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Name: Steve M.
Status: Other
Grade: Other
Location: CA
Country: United States
Date: September 2007


Question:
How can I find out the optimum highway cruising speed of my car to get the best gas mileage? I have been to dealerships and auto clubs, but no one can answer.



Replies:
Roughly speaking, the lower the speed, the higher the fuel efficiency (at highway speeds). Drag increases with speed, and it is a losing battle (e.g. doubling speed more than doubles the power input). For the vast majority of cars, 55 is more efficient than 65 is more efficient than 75, etc. So go as slowly as you are willing.

The rule of thumb does not extend all the way to zero MPH, so there is an optimum point. For many cars it is around 40-50MPH, but it depends heavily on type of car, load, road/driving conditions, etc. But 40-50 MPH is well below highway speed, so that is moot for this discussion.

The best answer though, is for you to be a scientist yourself. Go on a highway and figure it out for your car. Drive your car for a half-tank (or some easily quantifiable fuel amount) at 45MPH, 55MPH, 65MPH, and 75MPH (if legal) and see how far you can get on each. If your city has a loop highway, you can drive a long time without going too far from home. Experiment on a Sunday morning, or some other uncongested time. The loop also ensures equivalent paths and averages out wind direction. Try to pick days with similar weather, etc. Email us back with your results!

This is a common question, along with finding ways to increase efficiency at any speed. There are many, many factors that influence mileage. Google "car speed vs. fuel efficiency" and there you have enough reading to fill your whole weekend (or more). Some people do some pretty crazy things... I will let you read about that yourself.

Hope this helps,
Burr Zimmerman


Hi Steve,

The reason no one can give a clear answer is that there is no easy answer. The speed that gives the best gas mileage is dependent on many factors that are different for each car. Things that affect this are air resistance of the specific car, overall gear ratio, engine efficiency, rolling resistance, the exact type of tires used, and their inflation pressures, the viscosity of engine oil you use, and so on. The only way you can get an answer to this, is the tedious method of noting fuel consumption at various constant speeds and choosing the speed that gives the best results.

A cruise control generally promotes improved mileage, since it results in a more constant speed. The disadvantage, of course, is increased boredom.

Interestingly (and counter intuitively), accelerating overly slowly to cruising speed does NOT promote good mileage. In a car with a manual transmission, pressing the gas pedal half to three-quarters of the way to the floor when accelerating, and shifting at a low RPM (around 2500 RPM), surprisingly results in better mileage than (for example) taking 2 blocks to come up to speed since this method reduces "pumping losses" in the engine's intake tract.

In general, the speed range that results in the best mileage is around 50 MPH for many cars. But this is obviously not practical (or considerate to others) if you plan to do this speed on a freeway!

Regards,

Bob Wilson.


The reason you do not get many answers/information, is that the question of optimal speed for best gas mileage is very complicated. It is conceptually easy to ask, but difficult to answer practically. Search the term "optimum speed automobile" on any one of the major web search engines. You will find an abundance of 'hits' that go into the various factors in more depth. It is not just cruising speed that counts. But it would appear that ~55 mph is near the maximum for fuel efficiency, but the curve is very "mushy".

The whole topic would make an interesting class project. First , identifying what appears to be the most important factors, then breaking up into teams, and researching the various factors in more detail. This improves "searching" methods. Second, examining the way "models" are constructed to organize the data collected. One lesson to be learned here is the danger of unintentionally factoring the "answer" into the model. This is a real concern in constructing various models. Third, error progression -- How sensitive (robust) is the prediction to changes in the input? This aspect of "model" building is often ignored. If the model is not robust, you will get just about the same answer no matter how you change the input data, or alternatively, one factor increases while another decreases and together they cancel one another. Probably more answer than you wanted, but the topic is a good one to teach scientific research.

Vince Calder


Dear Steve M.,

I would be quite sure that you would get the best highway mileage at the lowest speed since wind resistance is a major user of energy at highway speeds.

I would think that at something below 25 MPH, the engine could conceivably become less efficient, though I really think 15 MPH is a more realistic boundary.

It is not easy for an amateur to measure. You might try keeping track of how many miles you go between filling your gas tank and exactly how many gallons it takes to fill it and then calculate your average MPG. You cannot, of course, travel down a highway at 15 MPH while burning a tank, but you could try going mostly over 60 (50?) and then mostly under 50 to see what a difference that makes.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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