Car Fuel Efficiency

I was under the impression that a car’s most fuel efficient speed was between 40 to 55 mph.

Recently, a news article suggested that best efficiency was around 60.

What is the most fuel efficient speed?

And if I may add to the OP’s question;

At what rate does an idling car consume fuel? I know there are many variables; but what would be a ‘rough’ figure?

The best fuel economy is going to be at the lowest speed you can go in high gear without lugging the engine. That used to be somewhere in the 45-55 MPH range for most cars, but newer cars tend to have steeper high gears these days, so it could very well be closer to 60 now.

Ballpark fuel consumption at idle for a typical car sized gas-powered engines is around a half a gallon per hour.

Not totally sure, but I think it depends on the aerodynamics of the car. The 45 MPH figure has been around for 30 or 40 years. Modern cars are more aerodynamic, so maybe 60 is a more accurate figure today.

I doubt that drag would decrease fuel efficiency at the same speed on a bullet shaped car as it would on a bus.

Jason’s answer sounds good to me.

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This doesn’t sound right to me. My car’s gas tank is 20 gallons, and by this math I should be able to drive for 40ish hours at optimum speed. That’s not realistic, and doesn’t jive with the 28mpg(highway) rating.

[quote=“GiantRat, post:6, topic:573874”]

That half-gallon/hour figure is for idling, which most cars do at about 750 rpm. Optimum speed of a car is at roughly double that rpm.

The engines I deal with use use a little less than 1kilogram of gasoline per hour* idling (more or less).
Out diagnostic equipment gives a read out of the amount of air mass ingested into the engine. This value is typically 13-15 kilos per hour. Since the engine runs at 14.7:1 air fuel ratio, an engine consuming 14.7 Kilos of air will burn 1 kilo of gas.
This value will go up for AC on or idling in gear.
*I’ll leave it as an exercise for the Air Canada fuel truck driver to convert kilos to gallons.

It depends on the vehicle, but most modern passenger cars are most efficient around the 50-55 mph range. This is a pdf of the DOE’s Transportation Energy Data Book; look at page 4-29 for fuel economy data as a function of speed. The most efficient speed has crept up over time, from ~40mph in the early 1970s to ~55mph in the mid-90s, which was the last time these tests were run.

You might think the most efficient speed has crept upwards even farther in the 15 or so years since the last DOE test, but these data from Consumer Reports from 2009 show a fuel economy decrease of about 11% when the speed is raised from 55mph to 65mph, which matches the average 1997 DOE data fairly closely.

Again, the actual speed for best fuel economy depends on the vehicle–aerodynamic drag dominates the losses at these speeds, so you would expect boxy vehicles to fare quite differently than sleek vehicles. However, the aerodynamic losses scale with vehicle speed squared, so increasing the most efficient speed is difficult because the actual power required at the wheels increases rapidly with speed. I wouldn’t really be suprised to see individual vehicles have best fuel economy at 60 or even 65 mph, but I doubt we’ll see much of an increase in the average best efficient speed of all passenger cars.

It really depends on the type of vehicle. 4 wheels drive pickup will get less fuel mileage than one of the same make and model 2 wheel drive with the same engine at the same speed.

That’s one of the things that makes some foreign made cars more appealing. Some of them can get better fuel mileage at 70 mph than other cars of the same size at 65 mph.

I’d add that it depends on the design of the car, the weight, the gearing and the engine. So it probably varies quite a bit between the various cars, with the more aerodynamic cars with relatively higher gearing favoring higher speeds. I used to have a Maxima that had an instantaneous MPG readout, and, according to that, 50 MPH was the best for MPG, (and, contrary to what some say, the AC taxed the engine 1.5 mpg).

Real world factors:

60 MPH gets your tranny in a safe range where it won’t hunt between top gear and the gear below. This is a problem when you are going 50. Even modest inclines induced a down shift. That is inefficient.

60 MPH also brings the engine RPM closer to a sweet spot, considering how tall (low numerically) most top gears are (overdrive).

In newer engines (lots of power per cc/CI of displacement) the engine is more efficient at slightly higher RPMs versus the old torque monsters.

Aerodynamics: This is not the answer. Two cars with identical aerodynamics and powertrains (including gearing), etc and the faster you go the more drag. I don’t think you are going to find many/any cars that have less drag at 60 vs. 50. In the case of aerodynamics, 60 is always worse.

I think the engine’s actual best (most efficient) operating point is somewhere mid-band in the RPM range, near where the torque peaks, and with the throttle almost wide open. Of course, you can’t operate very long in that region…

Graph here: http://en.wikipedia.org/wiki/File:Brake_specific_fuel_consumption.svg

Sure, if you’re looking for the most efficient way to get lots of torque. In a car that’s relevant for acceleration. But for cruising, you don’t need much torque to overcome rolling and air resistance. You just want to select the gear where the engine provides sufficient torque at the most efficient RPM. If I’m reading the diagram correctly, that’s somewhere between 1500 and 2000 RPM for the example you linked. And because the rolling resistance and air resistance increases with speed, the most efficient cruising RPM is probably closer to 1500 RPM at low speeds and perhaps 2500 RPM at highway speeds.

My calculations show about 0.2 gallons per hour.

I would submit to you that all things being equal (which they usually aren’t, granted), a more aerodynamic car would actually have a lower optimum speed. Because the fuel/air ratio is more or less constant, fuel consumption per hour is a direct function of engine speed (ignorning fuel-off coasting). So the only real “strategy” for increasing miles per gallon is getting more wheel revolutions per engine revolutions-- in other words being in the highest possible gear (and having the torque converter locked). The limitation to being in a high gear is “lugging” which is when the engine at a certain RPM can’t produce enough power to keep the car moving. If the car is more aerodynamic, it takes less effort to do this so you can drive slower in high gear and therefore your optimum fuel economy speed would be lower.

Now, in the real world cars that are aerodynamic are also going to be geared higher, so it is quite likely more aerodynamic cars might have higher optimum speeds, but it’s not a causal relation. If I put a parade float on my Lamborghini, I wouldn’t be able to go as slowly in high gear and so I would have to speed up a little to achieve optimal speed.

I think you have this backward. With a more aerodynamic car, your high gear can be taller without lugging, and give you a higher top speed at minimum RPM.

Or you could have a less powerful engine. Or you could leave it the same and have zippier accelleration. Or you could add more weight. My point is there’s no physical reason for correllation between aerodynamics and the optimum cruising speed-- it’s just that cars that tend to be designed with low coefficient of drag also happen to be geared steeper.

My car would be 42 mph which is the speed the torque converter locks up in high gear.

You are ignoring the throttle position. You are correct that the amount of fuel burned relates directly to the amount of air moving through the engine, but the amount of air moving through the engine is a function of both engine RPM and the amount of throttle opened. An engine turning a given RPM is moving much less air at 1/4 throttle than at 1/2 throttle, and is therefore using less fuel.