Determining the Maximum Fuel Efficiency

Ok, being lazy. You can make performance curves for any vehicle. This involves a straight line from some graph and a graph that is curved. Where this line is tangent to the curve at one point is the maximum fuel efficiency. I can’t recall the details of how this analysis is made and what exactly is on the axes. Was hoping that some bright person could explain this graph and hopefully someone could explain how to get these curves (they vary for different conditions) from a dealership or manufacturer. Is there a common speed that is near optimal for automobiles? I am thinking it is near 42 MPH. In any case I am curious to hear what people knwo about this.

You can determine the maximum fuel efficiency of the engine output, but that is not the same as the maximum fuel efficiency for moving the car (MPG)

There are a number of other factors to add to the equation. If you really want to know the answer to the real question it is best to get a car and do some careful real life testing.

You cannot simply calculate such a curve from engine data. Your data does not account for transmission, gear ratios, weight of vehicle, aerodynamics, or all of the other zillion factors that affect fuel usage.

Yet it is easy enough to determine optimum fuel efficiency for any car by road tests. All you need is an mpg gauge. Many new cars already have a factory-installed instrument. If not, you can install the one from ScanGauge as an add-on (

Drive a flat highway at various speeds and note the readings. Plot mpg vs. speed and you have your graph. The instrument need not even be accurate since all you want is maximum efficiency.

We can predict your result: you will obtain maximum mpg at the lowest speed that you can maintain in top gear. The idea of a graph is a good one, however. It may show you that you could increase your speed by 5, 10, or even 15 mph with little drop in efficiency, but that efficiency tails off sharply if you go any faster. Good luck.

Agree; the optimal gas mileage is the lowest speed in the highest gear with the engine running smoothly. That’s different for each vehicle and depends on gearing, torque curve, wind resistance, etc.

Don’t know where you got the 42mph, it’s a little too fast for most cars.

The mileage for CAFE purposes is done on a stationary lab stand by applying varying loads and driving cycles. It does not reflect real life situations.

When the government set the 55 mph limit way back they knew that for the square box cars then is use, any speed over 55 would really reduce mileage because of the air resisistance. The 55 mph limit was probably considered the lowest speed people would drive without developing road rage!

Your best fuel efficiency will likely be a dangerously low highway speed.

“The 55 mph limit was probably considered the lowest speed people would drive without developing road rage!”

Unfortunately they were off by about 20 mph. (-;

Any one here in auto design?

Ha, sorry living in Canada temporarilly and the speed limit is 50. 50 km/hr or 31.25 mph. This is killing my fuel efficiency. Miss the good old US with roads and speed limits. So it does matter on determining how much we want to speed.

Any one here in auto design?

What is it you mean with this question?
Aside from the 55 mph tangent, you got some very valid replies.

55 became the federally mandated maximum speed limit near the end of Richard Nixon’s presidency. Nixon proposed 50 mph for cars and 55 for buses. Congress chose 55 for everything.

A number of us are active members of the Society of Automotve Engineers (SAE)and/or qualified mechanics. You must have seriously bumped your head in Canada by trying to tell us that the speed limit is 50km/hr. That’s true within the city limits on streets. Like the US, and every other developed country, Canada has higher expressway and highway speeds, probably 100 km/hr or 60 mph.

As for 50 km/hr wrecking your fuel economy, you must have non-standard driving habits like leaving your car in 2nd or 3rd gear at that speed. You normally get better fuel mileage at 30 than at 60 mph.

I think the curve you are referring to is the Brake Specific Fuel Consumption, BSFC. These are curves that plot fuel burn rates as a function of RPM and load and is used by engine designers and engineers. These curves are only pertinent to the engine itself; the overall vehicle fuel efficiency is dependent on numerous other factors such as:
- weight
- drag coefficient
- rolling resistance
- frictional losses in the drive train, etc

Did not bump my head it was more of a serious multiple year infection from exposure to raw sewage in the public bodies of water, but thanks for asking. The speed limit where I expend 99% of my hydrocarbon energy is on municipal roads (which dominate the region) is 50 kph. So the number of interest is not like going on the expressway around DC where the speed limit is 70 MPH but you go 10. The Trans-Canada highways are 110 kph (69 mph). If the average maximum fuel efficiency were greater than 50 kph, then there is a good argument for having them officially raise the speed limit on the main arteries that are still considered municipal and still 50 kph. These are roads that would be classified for 45 mph in the US which would be great (they are similar to the road that I grew up on which was 55 mph in the US, it seems slower than is efficient).

The question is not do you get better fuel efficiency at 30 or 60. The question is for a given vehicle, how does one determine the performance curves such that an estimate of the best speed for fuel efficiency is attained. There are many factors that go into this, the engine wants to burn as hot as possible before melting down, the friction in the parts… Your assertion only deals with aerodynamics not with the mechanical systems. It is true that the power consumption goes as the cube of the velocity, however, at some point the gearing, friction, engineer performance and other factors will become more important in the force balance for equilibrium at your chosen cruise condition. So going slower is not always the answer. There is a maximum in there somewhere.

And I definitely get better gas mileage at 40 than 30. And, unfortunately, the car is automatic so we are not shifting it. At 50 kph, we get around 26 mpg, after injector cleaner on the highways averaging 70 MPH we get 34.5 mpg. Of course on the highway more time is spent at cruise and less (de)accelerating. Just trying to find the sweet spot for in town driving and devise a plan for replacing the vehicle.


Aircraft designers generate these curves and share them with potential customers. As many people have pointed out, the vehicle performance does not depend solely on the engine performance. In the aircraft industry, stand tests are accomplished similar to the performance curves that are mentioned here. But the aerospace engineers at the aircraft company design a plane around the engine specs, the aerodynamics, the structure, the dynamics. In the end, they produce a book of performance curves that illustrates the various performance characteristics of the plane.

When asking if anyone was an automotive designer, I was seeking people working for car companies (probably automotive engineers and SAE members) who could verify if such an analysis is done for cars. If this existed, then is it possible for a consumer to have a look at it. If so how. I was also seeking the basic format of the performance charts that deal with fuel efficiency and how to determine the maximum. As pointed out, one could perform road tests but that implies that you have already bought a car. It would be incredibly useful to map out where you are spending the most time (distance) in a car and then be able to choose a car that gives you the optimal MPG for that regime. I can see you buying a car and your partner buying a car where one was optimized for driving to work and the other was optimized for trips. Then you can optimize your total fuel efficiency when considering a black box surrounding the fuel you consume as a family v. the distance traveled to achieve your goals. To do this, performance curves would be incredible useful rather than looking at the sticker on the car window. It is rare to get the advertised fuel efficiency.

Yes the replies were valid, I assumed that they would be more about the vehicle rather than just the engine. Thanks for the initial replies, can we take this further?

Excellent. Can someone recommend a reference that covers fuel efficiency and design for the specific purpose of maximizing a vehicles fuel efficiency?

“Don’t know where you got the 42mph, it’s a little too fast for most cars.”

Is there a value that averages over passenger cars?

What is “CAFE”? Seems that an ideal efficiency at constant conditions gets worked out. Is this available? Choices can be made to drive when traffic loads are less. Do you have a reference?

55 became the federally mandated maximum speed limit near the end of Richard Nixon’s presidency.

Contrary to popular belief…The Feds can NOT set speed limits. Speed limits are STATE laws. What Nixon did was he told states that they would withhold federal funding for roads if they DIDN’T lower their speed limit to 55. And NH actually never LEGALLY lowered the speed limit. They just changed the signs.

That is very interesting. I had no idea that every car had this plug and ability to get data from the car. Is there a protocol on the wiring to the connector and the data streams that come out and how to parse them. It would be fun to plug a laptop into the car and collect the data. Probably could make a connector and program in a few weekends given the information. Then I could record a time series and really work some things out.

As Loafer points out, the actual fuel mileage has to be determined from level and steady speed road tests. Good car magazines used to do this for their readers, but found most readers were not interested.

For an individual car, as other posters pinted out, if it has a computer, driving on a level raod at various steady speeds will show the best gas mileage.

The EPA test are done in a lab, and some cars have been more off than others from the real life figures. Hyundai had perfectly legal CAFE gas mileage figures, but in real life they fell dismally short, a sign of sloppy design and power-robbing A/C for example. Hybrids have had very poor real vs calculated mileages.

Consumer Reports solves the problem by driving all their cars through a mixed loop of roads and traffic, and giving you for a couple of dollars, the best figures you will get anywhere.

Don’t ever assume that a car company will give you actual road gas milege figures! If you bought a fleet of 2000 cars for the government or private firm they probably would.

If you believe that a 'injector cleaner" did anything at all for your mileage you havent been reading the posts. Normal gasoline has addatives that clean your injectors. Injector cleaning is a scam and respectively you not realizing this puts a cloud of missunderstanding on the rest of your data

Yes, you are correct, there is a SAE design manual. I will look up the reference and send it to you.

Here is a description of the relevant SAE specs.