July 17, 2008

55 MPH to save fuel?


Recently US Senator John Warner suggested we return to a 55MPH national speed limit in order to save gas.

Would 55 MPH speed limit really save us gas? If so then how much?

The CNN article cited study that said 55 MPH would save 2% in national gas usage.

Its certainly true that lower high speeds will save gas. This article testing MPG versus MPH on metrompg.com has a number of graphs showing fuel usage at highway speeds. In every example the MPG dropped off fairly steeply compared to the MPH. New cars are not somehow miraculously more efficient to get better mileage at high speeds. The EPA graph shows a 20% increase in mileage dropping from 65MPH to 55MPH and the other graphs show mileage improvements in similar ballpark.

So it is true that mileage improves at 55MPH in general.

But while we would save gas is it going to save us overall? What are the other costs of a lowered 55 MPH national speed limit?? There are in fact other costs to a 55 MPH speed limit.

Lower speed limit takes more travel time which costs money. If you drop from 65 MPH down to 55 MPH then a 500 mile trip will take 9 hours, 5 minutes instead of 7 hours, 40 minutes. If your car gets 30MPG at 55MPH and 25 MPG at 65MPH then you'd save about 3.33 gallons of gasoline. At $4 / gallon that is a $13.33 savings for the 500 mile trip. But your spending an extra 85 minutes on the road to save that $13.33. The 85 minutes is worth around $8.28 at minimum wage.

It costs money to enforce a 55 MPH speed limit. The states will have to spend more on highway patrols to enforce the new speed limit.

I think there are better, more cost effective ways to lower our oil consumption.

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