Are internal combustion engines really most efficient at 55 MPH?

I’ve always heard that speed limits were originally set to 55 MPH because that is the speed at which automobile engines are most efficient. But is that really true? How is it possible that a Mazda’s rotary engine, the boxer in a Subaru, and a Ford V8 all share the same efficiency characteristics? Is this somehow inherent in the physics of the internal combustion engine or could you make a car that is most fuel effiecient at 75 MPH, or some other random speed?

I don’t think it is the engine itself. It is the wind resistance. The difference in wind resistance between 55 mph and say 75 mph is quite large because wind resistance increases as the square of speed or something like that. I have read that cars are most efficient at about 35 - 45 mph but then people wouldn’t ever get anywhere. 55 mph is the trade-off between reasonable speed and fuel efficiency.

I think the premise is wrong. Speed limits have changed any number of times over the decades. The 55 mph limit was never universal, even in the 70s after the gas crisis: many smaller freeways were kept at 50 for safety or traffic reasons. And the 55 mph limit applied only to federal interstates. States could set their own limits.

The 55 mph figure is simply a compromise between safety, design, gas consumption, and need to get places as quickly as possible. It does not relate directly to the cars themselves or engine efficiency. It is true that additional speed quickly erodes efficiency: about 40% at 70 mph IIRC. But each type of vehicle has its own characteristics and its own peak of efficiency.

According to the government,

The graph on the same page indicates that 45-55 mph is the most efficient range of speeds for most cars.

The maximum efficiency of the engine is the point when the fuel consumption curve crosses the performance curve. In other words, at some point you can make the engine go faster, but the gains are incremental compared to the power needed to get it to that point. That’s why a car gets better gas mileage at highway speeds, because you’re not wasting energy by braking and you’re not accelerating through the gears. This is generally some RPM level.

When you mate the engine to a transmission, you can use gear ratios to ensure that the engine is in the band of maximum efficiency at a certain speed. Naturally, for fuel mileage purposes it makes sense to put that target at or about the highway speed limit, because that’s where the average person does most of their driving. Therefore, if you’re doing anything but maintaining a constant speed for an extended period of time you’re not running the engine efficiently. You can change that point through gear ratios as much as you want to, but it wouldn’t make any sense. If you set that point for 35 for city driving you’ll absolutely kill your mileage on the highway, and you won’t see any gains in city driving either due to stopping and starting over and over again.

55 was set as the national speed limit to save gas during the 1973 oil crisis. Now, your engine may or may not be at its most efficient at 55, but would you agree that you’re using less fuel because the car is going slower? That was the idea. It had nothing to do with anything the automakers did or anything inherent to engine design.

Oh, yeah. When you start to factor in aerodynamics all bets are off. As others have said, at some point wind resistance really starts to kill you, but that’s a variable independent from the question you asked.

You design a machine to accomplish the task it is intended to accomplish. The most efficient automotive design for high speed long duration driving on roads with turns in both directions is a formula 1 car. For all turns to the left, it’s an Indy car.

The automobile should be designed to be most efficient at 55 mph because those are the conditions under which is most likely to operate. If the speed limit were higher, you would design for the higher speed limit.

Or, you could design for off road conditions, and loads of 10,000 lbs, and call it a hummer, and drive it at 55, empty.



55 was chosen because, at the time, many states had a top speed limit of 60, so imposing a national speed limit of 60 would not have raised consciousness in those states. They wanted to have an impact on the national conscoiusness that we have an energy crisis going on here.

That’s not even remotely true, unless you’re using “efficient design” to mean “best design” rather than fuel efficiency as the OP intended. The overriding design objective for Indy cars and F1 cars is not efficient operation. It’s to win races while staying within the boundaries of the imposed rules. The best design for winning races is the best tradeoff of a whole host of lesser objectives. Efficiency is one of those objectives, but other objectives (such as heavy acceleration) tend to drive the design in he opposite direction.

The automobile should be designed to be as efficient as possible at 55 mph. Different statement. Efficiency is dictated by power losses, which are a function of the operating conditions. You can’t arbitrarily choose to make the peak efficiency point at any speed you want–or, at least, it’s very, very impractical.

Those things aren’t exactly untrue, but your cause and effect is misleading. You’re mixing up (as does the OP) effects on engine efficiency and effects on vehicle efficiency. Energy loss due to acceleration/braking is indeed a big reason why city mileage is less usually less than highway mileage, but this is an effect on vehicle efficiency, not so much engine efficiency.

In fact, your engine is likely to be more efficient during acceleration than during steady-state running–engines, up to a point, are more efficient at higher loads.

Most states had at least a 65 mph speed limit at the time. The change was dramatic, but not strictly political. It was a sound conservation move that would be as important today.

I think this is it. I seem to remember being told that the point at which bugs start splattering on the windscreen instead of being blown over the car is the point at which you’ve exceeded your car’s “hull speed” if you like, and that’s where the fuel economy starts to get really bad. Your car is now having to punch air out of the way rather than slip through it.

55 is a compromise.
My simulator testing and real-life testing leads me to believe that the ideal speed for most cars is BELOW 55.
I’d say it’s somewhere between 35 and 50 MPH. The more brick-like the car, the closer to 35. The more streamlined, the closer to 50 MPH.
In any case, the difference between 35 and 50 MPH steady-state is small enough that you’re approaching the “splitting hairs” realm if you’re doing the test with a private vehicle on public roads and publicly available tools.
Even with a simulator, I have to question whether or not the tools are ACCURATE enough for the displayed difference between MPG at 40 MPH and 50 MPH to reflect anything besides noise in the equations and assumptions used.

My car gets 26 MPG on the way home from work at 55 MPH and 31 MPG on the way home if I cap my speed at 50 MPH.

I don’t think so - why would a land vehicle have “hull speed”? Air resistance is proportional to square of speed, and that’s a very good approximation from zero speed to nearly the speed of sound. Which means the slower the vehicle, the less energy required to travel the same distance. If real-life cars do not follow this trend, it’s only because the engine efficiency drops sharply at lower power output.

“Hull speed” for want of a better term, I guess. It’s probably more the point at which the exponential nature of the equation starts seriously to bite, which I’ve always been told kicks in at about 80kmh, which I think is about 50mph.

As I said, air resistance is proportional to square of speed. It’s not exponential, and it doesn’t suddenly “kick in” at a certain speed.

I think what TLD means is, “The speed at which bugs start splattering on the windscreen is a rule-of-thumb approximation for the speed above which drag losses substantially decrease a vehicle’s fuel efficiency.” He’s just using incorrect terminology.

IMHO 35 is too low for a modern transmission. at 35 it will not have shifted into its highest gear, and locked up the torque converter. 45-50 is probably closer to the best fuel mileage.

My Mustang can cruise comfortably at 35 mph in 5th, unless I hit a steep hill…

I think you guys are all off the mark.

First, stop and think about what the most efficient car would be - that would be the car with the lowest rolling friction and air resistance possible. Now let’s say a car like that needs 5 HP to maintain 55 MPH.

What would then the most efficient engine be? The most efficient engine would be one that makes 5 HP with a fully-open throttle, so there are no pumping losses. That combination, assuming a very efficient engine design in the first place, would be the ultimate vehicle for travelling 55 MPH.

But in the real world, we can’t accept this. First, we need an engine that makes more than 5 HP, because we need to accelerate better. Second, our car has to have larger, thicker wheels to absorb shocks and give us cornering traction. And the car will be heavier, and have a worse drag coefficient because it has to fit real people and carry luggage and be easy to get in and out of.

So we wind up with several curves - first, there’s the drag curve of the vehicle, which is basically an exponential curve. There’s no minima here other than at 0 mph - the faster you go, the more drag there is.

The second curve is engine efficiency, measured in Horsepower produced per unit of fuel consumed. A real-world engine will typically have far more horpsepower than we need for highway cruising, which means it will be operating somewhat inefficiently at 55 MPH, and less so at higher speeds.

So if you normalize the two curves, the intersection where they cross would be your most efficient speed. And it will be different for every vehicle - not just because of their shape and engine size, but because they are also geared differently, have different kinds of transmissions, etc. A very aerodynamically efficient sports car with a big engine might get the most mpg at a fairly high speed. On the other hand, a panel delivery truck with a smaller engine might have its most efficient speed way below 55 mph.

55 MPH is a compromise like others said, but I’m not even sure it represents the average efficient speed of the fleet. It was more likely arrived at politically, by determining what was feasible to get past the voters.