Do lower speed limits reduce gasoline usage?

There are three parts to this question:

• Does speed have an effect on miles-per-gallon? Is there a more accurate causal effect than speed that would also cause a connection at varying speeds? (perhaps RPM; weather; tire pressure)
• Are speed limits set at an appropriate point to directly affect miles-per-gallon? Should they be higher; lower? (Not to imply that gasoline usage is a relevant factor in speed limits.)
• Do drivers react to speed limits enough to matter? As in, do drivers obey speed limits enough to change their speed to one that improves their gas milage?

In addition, I would imagine that city and highway driving may have different results.

• I just saw your question on meta. I would say that part 3, at least, is a separate question: "Is there a strong correlation between speed limits and average driving speed?" May 23 '11 at 23:43
• @Renesis: That makes sense. For what it is worth, I started with this question and then realized that it had three parts while formulating the question. I don't personally think it is worth splitting all three of them up... but can you think of a way to only the first and second? And would the third be interesting in and of itself? May 24 '11 at 0:29

There are two very obvious reasons that higher speeds can reduce gas mileage:

1) drag and wind resistance: the faster you go the more the wind pushes back. Drag force scales with the square of velocity IIRC (and to overcome that force factors as a cube of the velocity) ( see: http://en.wikipedia.org/wiki/Drag_%28physics%29 )

2) vehicle tuning: cars get the best mileage in an optimal band, and many (non-performance) cars are set up so that optimal mileage in the highest gear coincides with the expected national speed limit (and EPA testing procedure for MPG stickers in the US).

see: http://www.fueleconomy.gov/feg/factors.shtml for a list of other common factors.

• The drag argument is somewhat misleading because the "squared" forces apply for a shorter period. Since work=F*t, the energy loss per distance travelled due to drag increases only linearly with speed. May 19 '11 at 13:38
• I admit that, but it is still a fact that it is not a strictly linear relationship wrt velocity. May 19 '11 at 14:43
• +1, great answer. This certainly addresses speed's relation to mpg. Do you think that vehicle tuning could (or does) overcome any mpg changes that would result in lowering the speed limit? In other words, would changing the speed limit overtly affect gasoline usage? May 23 '11 at 16:16
• gas usage is a factor of speed. the speed limit is not relevant. the question that ought to be asked is "how can we balance the needs of people with this ideal?" 65 mph seems to be the answer in much of the US, but the social answer is not really tied up with the science, so you'll never get a good answer May 24 '11 at 13:56
• @MSalters Actually this is not correct. Energy is force times distance, not force time time. As Wikipedia puts it "Exerting four times the force over a fixed distance produces four times as much work.". May 30 '11 at 18:38