# Saving Gasoline



## MrFSS

I recently returned from a driving trip to PA and MD (I know I should have taken the train - but it doesn't run where we live) and it was hot. In the 90's everyday and very humid, too.

For the first time in many years I noticed many, many cars on the interstate highways with windows rolled down as they cruised along at 60 - 70 MPH.

Do you think they are trying to save fuel by not running the A/C?

I had always heard that it is better to leave the windows up (less drag) and use the A/C rather than windows down and no A/C?

What's the answer?


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## the_traveler

There was just a news story on TV a few days ago, but more importantly a segment on Mythbusters  recently, about this very issue. It was determined that a car goes further with the windows up and the A/C on, than with the windows open, at highway speed!


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## PetalumaLoco

the_traveler said:


> There was just a news story on TV a few days ago, but more importantly a segment on Mythbusters  recently, about this very issue. It was determined that a car goes further with the windows up and the A/C on, than with the windows open, at highway speed!


I saw that episode and didn't like the results; 1 type of vehicle, 1 test, no variables such as ambient temperature.

Read this for a better answer.


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## saxman

Yeah, I heard using the AC has negligable effect on gas mileage. Unless its a nice 70 degrees outside, I'm going to use the AC. I'd rather be comfortable when it's 100 degrees here in Texas.

Has anyone else noticed this? Seems like people are driving slower down the freeway too. Like actually near the speed limit if its 60. Less 70 and 75 guys. I certainly have been going a little slower. My old Jeep is a gas hog.


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## MrFSS

saxman66 said:


> Has anyone else noticed this? Seems like people are driving slower down the freeway too. Like actually near the speed limit if its 60. Less 70 and 75 guys. I certainly have been going a little slower. My old Jeep is a gas hog.


Well - there must be an algebraic equation for this, too.

Think - I slow down and get better gas mileage but it takes me longer to get there and therefore I use more gas driving longer.

I go faster, using more gas, but I get there quicker and stop using gas sooner.

There has to be a magic speed number for optimal results of speed and time as it effects gas mileage.

Any math geniuses out there?


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## PetalumaLoco

It doesn't matter how long timewise your're driving, it's how many miles at what speed. You need to find the optimum speed at which you get the best MPG. 50 is always going to be better than 60 (or faster) because at higher speeds it takes more energy to fight the wind.


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## PetalumaLoco

If you want to see some higher math;



> The power required to overcome the aerodynamic drag is given by:
> P_d = \mathbf{F}_d \cdot \mathbf{v} = {1 \over 2} \rho v^3 A C_d
> 
> Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.
> 
> It should be emphasized here that the drag equation is an approximation, and does not necessarily give a close approximation in every instance. Thus one should be careful when making assumptions using these equations.


From Wikipedia; Drag Physics


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## Green Maned Lion

Every car ever made has a "cruising speed". That speed is the speed at which the car gets the best possible mileage. It varies heavily from car to car. It depends on engine size, power output, torque output, efficiency, gear ratios, and especially aerodynamics. A reduction in speed from 65 to 55 will increase mileage in most American built cars. (This includes, for instance, most Toyotas) This is because they are geared for relatively low speed. They are also geared to maximize mileage on EPA tests.

On the other hand, some cars would actually suffer mileage-wise. Mine would. Its cruising speed is about 72 miles per hour. It gets close to 38 mpg at that speed.

Opening your windows is a debate in which many people are misinformed. It also depends. First, it depends on the inherent aerodynamic efficiency of your vehicle- the lower it is, the less the effect. But the main thing is the speed at which you are moving. Around town, pottering around at low speeds, the windows being opened is more efficient. Let me tell you, there is no aerodynamic efficiency lost when you are sitting at a traffic light with your windows open. At highway speeds, leaving your window open is purely stupid.

Lastly, how cold do you really need it? Because ya know, your car can vent air without using the a/c. And leaving the windows up and A/C off produces the best mileage by far. Most cars have an Economy feature. Try it, you might be surprised.

PS: Make sure you use your A/C atleast once a week during the summer. Otherwise you are asking for the system to fail.

Want another key tip to better efficiency? It amazes me how many people don't get this. Brake Hard, Accelerate Soft. Meaning, attempt to save as much speed as possible in most situations. It saves fuel when you don't have to accelerate. Take corners with as little braking and as little gas as you are comfortable with. Most fuel is wasted when the car is attempting to accelerate. Drive to avoid doing so.

If you must accelerate, do so softly. There is no drag race to be won between two red lights. Lift your foot off the gas to make the car upshift as much as possible. If you drive a stick, start the car in second or, if it can handle it, third gear. As soon as you can, shift into top. As soon as you can means about 200 RPM above stall.

Also, when it comes to mileage, cruise control is your friend. I don't care how good you are, it will maintain speed more accurately than you and will reduce accel/decel losses.

Oh, one more thing. If you find yourself going faster than you are looking for, don't brake. lighten your foot on the gas and let it slowly move down. Putting your foot in a position for decelerating to 60 takes no more gas than going 60.


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## WhoozOn1st

Green Maned Lion said:


> Most fuel is wasted when the car is attempting to accelerate. Drive to avoid doing so.


A brilliant concept! Why didn't I think of that? So simple! The answer to our fuel hassles is to never accelerate. Forget the fact that you'd never get anywhere, and that you'd be burning fuel at idle while going nowhere. GML is truly a genius, and I nominate him for the Nobel Prize for energy. Unfortunately he won't be able to claim the prize in person, as that would require acceleration of some sort.


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## Neil_M

WhoozOn1st said:


> Green Maned Lion said:
> 
> 
> 
> Most fuel is wasted when the car is attempting to accelerate. Drive to avoid doing so.
> 
> 
> 
> A brilliant concept! Why didn't I think of that? So simple! The answer to our fuel hassles is to never accelerate. Forget the fact that you'd never get anywhere, and that you'd be burning fuel at idle while going nowhere. GML is truly a genius, and I nominate him for the Nobel Prize for energy. Unfortunately he won't be able to claim the prize in person, as that would require acceleration of some sort.
Click to expand...

:lol: :lol:

Of course, seeing as he never tires of telling everyone he weighs 600lbs or something, surely the best way to save fuel is to take GML out of the car...........


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## transit54

Green Maned Lion said:


> Want another key tip to better efficiency? It amazes me how many people don't get this. Brake Hard, Accelerate Soft.


Unless you're driving a hybrid. Then accelerate soft and brake slowly, long and softly for as much energy regeneration potential as possible.


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## George Harris

"Brake hard" as a way to save gas and wear and tear on your car has got to be one of the dumbest things I have heard in a long time.

The key is anticipation of what you will have to do. brake lightly or not at all. Let it slow with foot of the gas as far as possible. Your brakes and tires will last longer. There is a lot more to the cost of running a car than just gas.

The best way to save gas is to combine as many functions into one trip out as you can. There was a WW2 ear poster concerning riding trains which said, "Is this trip necessary" If you ask yourself that one before jumping in the car and decide to not go at all or wait until you can combine what your are going for with another trip you have to make, that will save more gas than all the other tricks in the book.


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## WhoozOn1st

George Harris said:


> "Brake hard" as a way to save gas and wear and tear on your car has got to be one of the dumbest things I have heard in a long time.


Full agreement.


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## jackal

PetalumaLoco said:


> If you want to see some higher math;
> 
> 
> 
> The power required to overcome the aerodynamic drag is given by:
> P_d = \mathbf{F}_d \cdot \mathbf{v} = {1 \over 2} \rho v^3 A C_d
> 
> Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.
> 
> It should be emphasized here that the drag equation is an approximation, and does not necessarily give a close approximation in every instance. Thus one should be careful when making assumptions using these equations.
> 
> 
> 
> From Wikipedia; Drag Physics
Click to expand...

I did know this (that drag goes up exponentially with speed), but then does that mean that the most efficient speed is idling (in top gear, I'd guess)? Is 15mph more efficient than 35mph? I thought I've heard that the most fuel-efficient speed is around 45mph (give or take 10mph), although I'm not entirely sure why that would be (maybe that's approximately the peak of the power curve of a typical automobile in top gear).


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## VentureForth

Sort of why we went to a National speed limit of 55 in the Carter Crisis. It was determined to be the most fuel efficient speed.

As for braking, brake softly well in advance of a stop light. That way you minimize your stopped time and reduce acceleration when the light turns green.

All the best driving techniques will only get you one or two MPG, though.

Just buy a GEO METRO!


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## MrFSS

My daughter is a mechanical engineer for Firestone Industrial Products. As such she is involved in vehicle design, among other things. I put the following question to her:

*Is there a magic speed number where a car will get the best cost for a particular trip. *

* *

*i.e - suppose all things are equal and two cars travel the same 500 mile route with the same conditions and equipment. *

* *

*The first car goes an imaginary 100 MPH and it therefore take about 5 hours. *

* *

*The second car drives it at 50 MPH and will take about 10 hours. *

* *

*Now, the second car gets better gas mileage because of the slower speed, but is using the engine twice as long as the first car. So, the first car, while not getting good mileage gets there quicker and only runs its engine half the time the second car does. (make sense I hope) *

* *

*There has to be a way to figure a magic number for a particular car going a set distance as to what its speed should be to achieve the best/least amount of gas used for the trip.* 

And, her response was:

*The first car going 100 MPH will chew through a TON more fuel than the one traveling at 50 MPH to cover the same distance. *

* *

*The crux of the question is how to minimize the amount of energy (fuel) needed to push a given mass (the car) a given distance (500 miles). While the first car does use the engine for half the amount of time, that is offset by several factors. The first one, and probably the biggest issue, is that it takes a lot more energy (fuel) to push a car at 100 MPH for 500 miles. The wind resistance is, in part, a factor of the drag coefficient of the car, multiplied by the square of the velocity. So as velocity increases, the drag increase is substantial. The net result is that you have to mash the accelerator a lot more to keep the engine bubbling along at whatever RPM is required to hold a particular speed. So, with all else being equal, a car running at 100 MPH is spinning twice as many RPM in the engine as one running at 50 MPH (for arguments sake, let's say 4000 RPM at 100 vs 2000 RPM at 50). In a frictionless world, one without any wind resistance, the cars' fuel consumption would only be variable by the cars BSFC (Brake specific fuel consumption, more on that later). However, with the wind resistance, the car's engine needs to produce a lot more horsepower (and torque) at 4000 RPM to keep pushing the car through the air. For instance, if it takes only 50 HP to overcome the wind at 50 MPH, it may take 150 HP to keep the car steady at 100 MPH. *

* *

*To expand a bit further, a car running at 50 MPH into a headwind of 10 MPH will use more fuel than a car at 50 MPH with no headwind. The engine is running at the same speed, but the engine needs more fuel to overcome the wind resistance, so you're giving the car more gas. *

* *

*As far as ideal conditions, it's very difficult to itemize. When an engine is dynoe'd, you can get the BSFC (Brake specific fuel consumption). This basically gives you the engines fuel economy for every engine speed (RPM) and torque output. If you run the engine at peak efficiency, where the BSFC is optimized, then theoretically you're getting the best fuel economy. This is generally optimized at around 35% of maximum RPM, and 75% of the peak torque for unmodified passenger cars. In terms of what that means with the throttle, it suggest "bogging" the engine for city driving (if you have a manual). Offering more throttle at lower RPM will raise the torque output, but keeping the engine RPM at less than 35% of redline should keep the fuel consumption more moderate. *

* *

*At highway speeds, it means about the same. Keep the engine RPM below 35% of redline RPM. Exceeding that means the engine is getting a greater thirst for fuel for every increment above that. Coupled with the fact that the wind resistance is also increasing as engine speed increases, you get a double-whammy and an overall much thirstier car. *

* *

*(As an aside, I'm not sure exactly how the BSFC is calculated or measured. I believe that the engine's volumetric efficiency has a little to do with this, as well. The volumetric efficiency is how much air/fuel each cylinder is actually taking in on each engine revolution. in a 3.0 liter v-6, each cylinder has a volume of 0.5 liters. However, on the intake stroke, the cylinder cannot take in that much air/fuel. Turbulence in the intake charge, temperature and pressure variations, intake charge velocity all take part in the volumetric efficiency. Lower volumetric efficiency generally means less power produced for a given engine revolution. Volumetric efficiency is generally highest (best) near the engine's peak torque RPM. Volumetric efficiency is basically a measure of how efficiently an engine is using the available engine displacement at any given time. *

* *

*So, the ideal realm to operate a car's engine, very generally speaking, would be to have the engine operating at or near the optimized BSFC and torque. However, if this peak requires running the car at a speed where the wind resistance begins to offset any efficiency gains in the engine, then you're still a net negative on fuel economy for the car. Let's say the best BSFC is around 2200 RPM. Ideally, you'd want to operate the engine here. But, if this ratchets the speed to a point that wind resistance is out gaining efficiency increases, you'd be better off running the engine at a less optimal 2,000 RPM. *

* *

*So, to find the "optimum" for any car would be very difficult. We'd need a BSFC map for the engine, the overall gearing (to know what vehicle speed a given engine RPM will produce), know the drag coefficient, and probably a litany of other items I'm overlooking. *

* *

*But if it was me, I'd try to keep the engine at or below 35% of redline RPM (2100 RPM for a 6000 RPM redline) to get the best bang for my fuel buck. *


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## PetalumaLoco

MrFSS

Excellent, I enjoyed your daughter's answer.


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## Green Maned Lion

WhoozOn1st said:


> Green Maned Lion said:
> 
> 
> 
> Most fuel is wasted when the car is attempting to accelerate. Drive to avoid doing so.
> 
> 
> 
> A brilliant concept! Why didn't I think of that? So simple! The answer to our fuel hassles is to never accelerate. Forget the fact that you'd never get anywhere, and that you'd be burning fuel at idle while going nowhere. GML is truly a genius, and I nominate him for the Nobel Prize for energy. Unfortunately he won't be able to claim the prize in person, as that would require acceleration of some sort.
Click to expand...

Very witty. What I meant was obvious, however.

I can see that my comment on braking came across differently than I intended it to. By "Brake hard", I meant, more or less, to avoid braking when it is possible to do so. Likewise, avoiding accelerating because you didn't brake and therefore don't need to re-accelerate. It also would have been better, in retrospect, to explain that by avoiding accelerating, I also meant avoiding hard acceleration.

The comment about RPMs makes quite a bit of sense, but fails to consider certain standard production engines whos RPM characteristics are far off the norm. I personally don't suggest anyone consider running a Honda S2000 or current BMW M3 at 35% of redline with a hope towards any kind of fuel economy. Both of those cars have redlines over 9000rpm. On the other hand, a Mercedes OM617 has a redline of 3100rpm, and running it at 35% of that (1085 rpm) would not be a wise idea, either.


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## PetalumaLoco

Green Maned Lion said:


> The comment about RPMs makes quite a bit of sense, but fails to consider certain standard production engines whos RPM characteristics are far off the norm. I personally don't suggest anyone consider running a Honda S2000 or current BMW M3 at 35% of redline with a hope towards any kind of fuel economy. Both of those cars have redlines over 9000rpm. On the other hand, a Mercedes OM617 has a redline of 3100rpm, and running it at 35% of that (1085 rpm) would not be a wise idea, either.


I believe that was covered with this statement;



> So, the ideal realm to operate a car's engine,* very generally speaking*, would be to have the engine operating at or near the optimized BSFC and torque.


(my emphasis)


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## gswager

On my rural highway (55 mph), there are three sharp curves (30ish mph) which forced me to slow down. Brakes? No, I never used it because I allowed to let my accelerator pedal go to allow coasting (Hey, that's 99 mpg!) ahead of time. The drag/friction will slow my vehicle down. If I'm running with A/C on, I had to plan ahead of time than without A/C on, due to high idle.

I've seen a lot of vehicles hit their brakes just before the curves.


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## jackal

gswager said:


> On my rural highway (55 mph), there are three sharp curves (30ish mph) which forced me to slow down. Brakes? No, I never used it because I allowed to let my accelerator pedal go to allow coasting (Hey, that's 99 mpg!) ahead of time. The drag/friction will slow my vehicle down. If I'm running with A/C on, I had to plan ahead of time than without A/C on, due to high idle.
> I've seen a lot of vehicles hit their brakes just before the curves.


Me? I'd just take the curves at 53mph... 

MrFSS, thanks for asking your daughter that. Extremely informative answer. I'm saving it to my clippings file. Now, just to determine what the BSFC for a 1997 Nissan Altima is... :lol:


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## MrFSS

jackal said:


> MrFSS, thanks for asking your daughter that. Extremely informative answer. I'm saving it to my clippings file. Now, just to determine what the BSFC for a 1997 Nissan Altima is... :lol:


I'll pass the word to her. She doesn't get it from me. She was the type that was doing calculus and differential equations in the 7th grade and had graduated to advanced thermodynamics by the 9th grade. When she graduated from high school she also had her 1st year of college done. Got to love those Boilermaker Grads!!!


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## Joel N. Weber II

jackal said:


> gswager said:
> 
> 
> 
> On my rural highway (55 mph), there are three sharp curves (30ish mph) which forced me to slow down. Brakes? No, I never used it because I allowed to let my accelerator pedal go to allow coasting (Hey, that's 99 mpg!) ahead of time. The drag/friction will slow my vehicle down. If I'm running with A/C on, I had to plan ahead of time than without A/C on, due to high idle.
> I've seen a lot of vehicles hit their brakes just before the curves.
> 
> 
> 
> Me? I'd just take the curves at 53mph...
Click to expand...

This also depends to some extent on what car you're driving. I'm pretty sure it's possible to find examples of curves where some cars can comfortably go through the curve 10 MPH faster than some other cars. Of course, the local law enforcement officials may not be terribly interested in paying attention to that distinction, and if you're driving in heavy traffic, you may need to slow for the car in front of you that needs to take the curve more slowly (on the other hand, in the case of a rural highway, that heavy traffic may not be likely).

I tend to slow to 10-15 MPH, probably by taking my foot off the pedals, a couple tens of car lengths before a red light when I'm driving at all. I figure that slowing all the way down to a stop and then accelerating is less efficient than keeping my minimum speed at 10-15 MPH the whole time. And if there are enough cars waiting at the light, I can often pick a time to accelerate such that I'll catch up to the last car stopped at the light shortly after it starts accelerating.


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## PetalumaLoco

> and if you're driving in heavy traffic, you may need to slow for the car in front of you that needs to take the curve more slowly...


Think of how efficient it would be if you never had to slow for the vehicle in front of you. How? By connecting any cars traveling in the same direction together. Matter of fact, you could have the lead car doing all the work, and the following cars with engines off in neutral. Man, what a great idea. Sounds kind of familiar though... <_<


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## jackal

Joel N. Weber II said:


> jackal said:
> 
> 
> 
> 
> 
> gswager said:
> 
> 
> 
> On my rural highway (55 mph), there are three sharp curves (30ish mph) which forced me to slow down. Brakes? No, I never used it because I allowed to let my accelerator pedal go to allow coasting (Hey, that's 99 mpg!) ahead of time. The drag/friction will slow my vehicle down. If I'm running with A/C on, I had to plan ahead of time than without A/C on, due to high idle.
> I've seen a lot of vehicles hit their brakes just before the curves.
> 
> 
> 
> Me? I'd just take the curves at 53mph...
> 
> Click to expand...
> 
> This also depends to some extent on what car you're driving. I'm pretty sure it's possible to find examples of curves where some cars can comfortably go through the curve 10 MPH faster than some other cars.
Click to expand...

On rural highways, I usually aim for about 10-15mph over the posted yellow warning--i.e. if the warning speed for the curve is 45mph, 55 to 60 feels completely safe--I feel centripetal/G forces pushing me to the side, but the car maintains complete control. (I think the posted warnings are to minimize the sideways forces passengers feel for a more comfortable ride--the car itself can handle much more if you don't mind the side-to-side forces.)



Joel N. Weber II said:


> Of course, the local law enforcement officials may not be terribly interested in paying attention to that distinction.


If the speed limit on a rural highway is 55mph and you come across a yellow warning sign for a cuve with a speed limit of 40mph, from what I've been able to gather online, you can still legally do 55mph through the curve. Yellow warning signs are not legally enforceable speed limits--only black-on-white speed limit signs are legally enforceable. (Same goes for the yellow-on-black

Many people have posted online that at the very most, a cop who would normally let you get away with 5-10mph over the posted speed limit would not, if you took a curve at a speed higher than the legal limit, give you any leeway. In other words, if the speed limit is 55 and you took the curve at 56, the cop would give you a ticket for 1mph over.



PetalumaLoco said:


> Matter of fact, you could have the lead car doing all the work, and the following cars with engines off in neutral.


I'm not sure a single car's engine would be able to haul all the following cars. BUT, if every couple of cars did engage their transmissions, and of course you'd want to use diesel to maximize fuel efficiency, I've thought of a great name for such a contraption: how about calling it a Diesel Multiple Unit? :lol:


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## the_traveler

jackal said:


> I'm not sure a single car's engine would be able to haul all the following cars. BUT, if every couple of cars did engage their transmissions, and of course you'd want to use diesel to maximize fuel efficiency, I've thought of a great name for such a contraption: how about calling it a Diesel Multiple Unit? :lol:


Or maybe *Amroad*! :lol: (No, I think that name is taken already!)


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## Joel N. Weber II

PetalumaLoco said:


> Think of how efficient it would be if you never had to slow for the vehicle in front of you. How? By connecting any cars traveling in the same direction together. Matter of fact, you could have the lead car doing all the work, and the following cars with engines off in neutral. Man, what a great idea. Sounds kind of familiar though... <_<


It doesn't quite work that well for trains. The EMU subway system I frequently ride sometimes has trains sitting around on the tracks between Davis and Alewife, waiting for the next train to depart Alewife to free up platform space.


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