Does the optimal speed for fuel efficiency vary by car model?

July 11, 2008 7:24 AM

What is the optimal speed for fuel-efficiency for my car? Do I really have to drive 55?

As I've been listening to the increasing buzz about resurrecting a national 55 MPH speed limit, I starting thinking about where that number came from, the types of automobiles in use at the time the number was derived, and whether it is still applicable today.

From what I understand, above 55 MPH is the point at which wind resistance increases drag enough to cause a disproportionate increase in the amount of fuel consumed in order to move the car a certain distance. Of course, that was what we were told in the 1970s when people were driving Lincoln Continentals and El Caminos. Cars today are supposed to be engineered to be more aerodynamically sound, thus less drag.

So my questions are these:

- Does the optimal speed for fuel efficiency vary by car? Will my Honda Fit be just as efficient at 65 than at 55, where a Land Rover won't?

- Are there any sources of data (tables, etc.) by car model that would have this data?

- Are there other factors that I'm missing besides, wind resistance, that contribute to fuel inefficiency at speeds greater than 55 MPH?

posted by scblackman to travel & transportation (18 comments total)

From the Fed

While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.

You can assume that each 5 mph you drive over 60 mph is like paying an additional $0.30 per gallon for gas.

posted by drezdn at 7:29 AM on July 11, 2008

Yes, but the point of my question is: how was that data derived? If it was originally derived in the 1970s with big, less-aerodynamic cars, is it not valid for today's modern aerodynamic car?
posted by scblackman at 7:31 AM on July 11, 2008

I don't have any hard evidence, but if anecdotal evidence is permissible:

In early June, my wife and I drove to Cape Cod (from Delaware) in our ('07 Automatic Transmission, Sport model) Honda Fit making extensive use of the ScanGauge (highly recommended!) and monitoring our fuel efficiency.

We found that anywhere between 50 and 65, we seemed to get quite similar fuel efficiency with a slight preference for the low end of that range.

As a reference point: over the total 400 miles, we averaged 39.5 mpg, with a several hour period in which we average 41 mpg.
posted by JMOZ at 7:35 AM on July 11, 2008

Obligatory Car Talk answer.
posted by TedW at 7:46 AM on July 11, 2008

Are there other factors that I'm missing besides, wind resistance, that contribute to fuel inefficiency at speeds greater than 55 MPH?

Rolling resistance, including not just tires but wheel bearings.

Also, the fact that cars are more aerodynamic now means they are more efficient at any given speed, but since the drag coefficient and frontal area are constant for any car while drag varies as the square of the speed, for every 1.4 times increase in speed the drag will double, according to this equation:

There is a basic equation for the force it takes to push something through air:

Aerodynamic drag = 1/2 D x A x Vsquared

In this equation, D is the density of the air, A is the frontal area of the moving shape, and V is its velocity relative to the air.

For real body shapes, air at standard conditions, V in mph, and drag in pounds of force, this equation becomes:

Drag = 1/391 x Cd x A x Vsquared

This equation shows that to calculate drag you need to know three things: Cd, the drag coefficient; A, the frontal area of whatever you’re driving through the air; and the speed of air past it. This equation shows an important point—aerodynamic forces are proportional to the square of the speed. That means you quadruple the drag or lift when you double the speed.

From here.
posted by TedW at 7:56 AM on July 11, 2008

Check out your owners manual. Sometimes they'll give you a range of speeds or target RPM where you get the best gas mileage.

As my anecdote: I recently rented an 08 Hyundai Elantra to drive from Houston to San Antonio. I set the cruise at 75mph and used the A/C the entire way. After refilling up I calculated the milage out to 37.5 mpg, 0.5 mpg better than advertised.

I have a feeling most car manufacturers have been designing cars to get peak economy at highway speeds of 65-70 mph, but I don't have any data to back it up besides my own anecdotal experiences.
posted by sanka at 8:00 AM on July 11, 2008

The Drag Coefficents for various cars are available online.
posted by drezdn at 8:00 AM on July 11, 2008

The Car Talk guys have it right, as they always do. Your fuel efficiency is highest when you're running the engine at the lowest speed you can maintain in your highest gear. Accelerating from there will lower your milage, no matter what type of vehicle.
posted by echo target at 8:04 AM on July 11, 2008 [1 favorite]

In my experience incorrect tyre pressure has a far bigger impact on efficiency than speed. But that could be down to my particular car and driving habits.
posted by le morte de bea arthur at 8:08 AM on July 11, 2008

There were complaints when the speed limit was lowered from 65 mph to 55 mph that it could lower, instead of increase fuel economy, and in fact the 1997 Toyota Celica gets 1 mpg better mileage at 65 than it does at 55 (43.5 vs 42.5), although almost 5 mpg better at 60 than at 65 (48.4 vs 43.5). Other vehicles tested had from 1.4 to 20.2% better mileage at 55 mph vs. 65 mph.[10]

Two questions for someone who understands cars better than I do... Could you find the ideal sweet spot for fuel efficiency by slowly getting up to the highest gear and then finding the speed at which you get the lowest tachometer reading? Secondly, would it be possible (through gearing perhaps) to make the engine most efficient at a higher speed?
posted by drezdn at 8:09 AM on July 11, 2008

I'm going to disagree (but only slightly) with the lowest-rpm-is-always-best notion.

Take a look at this article. There's a lot there, but the take-home message is that, cruising under light-to-moderate load, you do best on fuel not at super-low RPMs, but near the engine's torque peak. (At full load, economy is fairly flat.)

Your Fit looks like it has a fairly flat torque curve above 2300 RPM or so. What this means is that, for ideal fuel efficiency at highway speeds (assuming you're not going uphill or driving into the wind or towing a Radio Flyer), you want to be somewhere in the 2500rpm range in top gear. What speed that translates to I can't say without doing math, so try it for yourself.

The flip side, of course, is that, as this Corvair guy notes at the end of his response, these economy cars have tiny engines that are under moderate-to-heavy load at highway speed regardless of whether it's 55 or 65, so your efficiency won't change a whole heck of a lot.

So, keep the revs low, but not too low.
posted by uncleozzy at 8:16 AM on July 11, 2008 [2 favorites]

Yeah, that's something to consider...diesel engines are most efficient at or near idle, but gas engines are not, so the cartalk theory of keeping the engine lugged...um, not so much.
posted by notsnot at 8:43 AM on July 11, 2008

drezdn: Could you find the ideal sweet spot for fuel efficiency by slowly getting up to the highest gear and then finding the speed at which you get the lowest tachometer reading?

Your wheels are mechanically coupled to your engine (in the case of automatics, this isn't exactly true, but the principle holds). In a given gear, the faster the wheels are turning, the faster the engine is turning. At no point will your wheels speed up and your engine slow down, except when you shift. However, the amount of gas it takes to keep the engine firing at a given RPM can vary depending on circumstances. If you've ever driven up and down a hill, you've noticed this.

Secondly, would it be possible (through gearing perhaps) to make the engine most efficient at a higher speed?

Yes. Imagine if you only had first gear. You'd be way past redline at highway speed. Very inefficient. The trend towards adding gears (I think some Mercedes automatics now have 7 speeds) is partly to permit the engine to run in its sweet spot at any speed. If the engine is most efficient at the lowest speed where you're not lugging it, or the lower end of the torque plateau, or whatever, the engineers can tweak gearing ratios (and/or final-drive ratio) so that the engine will achieve that RPM at 50 mph, or 75, or whatever.

Wild speculation: it seems possible that a car could be designed to be more aerodynamic at 70 mph than 50 (leaving aside gadgets like skirts that deploy automatically). Laminar flows can abruptly change to turbulent flows above certain speeds, and I wonder if the reverse ever happens—as I understand it, pickups are designed to create air bubbles over their beds, smoothing out the airflow, and I'm guessing this only happens above a certain speed. I also wonder if it would be practical to design for it.
posted by adamrice at 8:53 AM on July 11, 2008 [1 favorite]

If it was originally derived in the 1970s with big, less-aerodynamic cars, is it not valid for today's modern aerodynamic car?

Some cars have near-instantaneous electronic gauges that give feedback on mileage. And I've driven fairly new cars where this problem very much remains the case.
posted by astrochimp at 9:10 AM on July 11, 2008

People are talking about two different types of efficiency in this thread: the efficiency of the engine and efficiency of the car as a whole. The engine is most efficient at producing power when it is running at its peak torque speed. However, the efficiency of the whole car depends on the efficiency of the engine plus the air resistance, the rolling resistance, power loss in the transmissions, and so on.

The air resistance is the most important factor at highway speeds and we all know that the air resistance is proportional to the square of the speed. While in reality, it isn't quite that simple, it's not a bad approximation. So naively we would think that efficiency would go down as speeds go up, but the engineers who designed your car took into account typical highway speeds while designing the car and designed the gearing so that typical highway speeds are achieved at (very roughly) peak engine efficiency. This fact makes up somewhat for the lower efficiency of high speeds due to air resistance.

So, yes, it is possible that your car would be more efficient at a speed higher than 55mph, but that isn't a property of the aerodynamics of your car, but of the gearing of the car. You could make it more efficient at 55mp by changing the gearing.
posted by ssg at 9:10 AM on July 11, 2008 [1 favorite]

the engineers who designed your car took into account typical highway speeds while designing the car and designed the gearing so that typical highway speeds are achieved at (very roughly) peak engine efficiency.

Agreed. All this talk of 'lowest speed in the highest gear' is completely wrong - for diesels or petrols, this just doesn't make sense. No engines are at their most efficient at the bottom of their rev range. There will be a balance point between drag and the ability of the engine to cope with it. I'm not sure if there is an easy way for it to be calculated for your own car, though. Somewhere between 55 and 70 mph is likely to be the place to start, and instant mpg gauges are the easiest way to find out. Another way is to, the next time you do a long run, fill up your car when you get on the highway and then again before you come off. If you manage to keep a steady speed over a decent distance (Like most of a tankful) that will give you an idea, but it will obviously take several trips to get an accurate answer. Knowing the coefficient of drag for your car will only help if you can somehow calculate the effective energy to counteract it from the engine, which will involve calculating all the losses from the transmission, wheel bearings, tyres (at a certain pressure) and all kinds of fudge factors. A nightmare of potential inaccuracy and requiring assumptions or lots and lots of measuring for a theoretical number - practical experiments may be a lot easier.
posted by Brockles at 8:29 PM on July 11, 2008

My answer above (for your car, as it were!) is the result of the practical experiment, but be forewarned: the instant MPG number is practical impossible to use, as it varies quite rapidly with even very minor changes in throttle position. You might consider the average over periods of time, but even this is affected drastically by traffic, hills, wind, etc.

Although everyone is correct that the lowest speed in the highest gear isn't the right thing to do, the drive-by-wire throttle position on the Honda Fit makes it hard to drive at much less than 2000 rpm, anyhow, making it easy to be in the right place. The big key (even on highway) is very slow acceleration. Whether you accelerate to 55 or 70 doesn't make much of a difference based on my measurements. (~1-2mpg difference for the Honda Fit, with the peak somewhere in the neighborhood of 55-60 mpg on flat ground).
posted by JMOZ at 5:13 AM on July 12, 2008

Brockles is correct. It takes a certain amount of power to maintain a certain speed. Every engine, transmission and wheel combination will be different. A gasoline engine is spectacularly non-linear in it's fuel to power curve- check out this article for more info.

Also note that maintaining speed on a highway takes relatively little power. The engine will be at something like 10% load. So you'll probably be under the curve, not over it.
posted by gjc at 9:55 AM on July 12, 2008