Driving 1 mph slower will improve your gas mileage

It's no myth: Driving 1 mph below the posted speed limit will significantly improve your gas mileage, especially over 55 mph. 

Joshua Lott/Reuters/File
Fuel prices are displayed at a Chevron gas station in Phoenix, Arizona. It's no myth: You truly can save significantly on gas just by going one mph slower than the posted speed limit.

Saving Pennies or Dollars is a new semi-regular series on The Simple Dollar, inspired by a great discussion on The Simple Dollar’s Facebook page concerning frugal tactics that might not really save that much money. I’m going to take some of the scenarios described by the readers there and try to break down the numbers to see if the savings is really worth the time invested.

Gayathri writes in: Driving 1mph slower than posted speed limit. Yeah, that’s a myth.

Actually, it’s not a myth. Most cars made in the United States maximize their fuel efficiency at about 55 miles per hour and drop off rapidly above that limit (this is actually from a study – West, B.H., R.N. McGill, J.W. Hodgson, S.S. Sluder, and D.E. Smith, Development and Verification of Light-Duty Modal Emissions and Fuel Consumption Values for Traffic Models, Oak Ridge National Laboratory, Oak Ridge, Tennessee, March 1999).

This means that if you’re tooling along on the interstate at the speed limit of 65 miles per hour and drop that back to 64 miles per hour, you’re actually improving your gas mileage by about 1.5%, according to fueleconomy.gov.

So, let’s work out what that’s really worth.

Let’s say you have a typical car that gets 25 miles per gallon at 55 miles per hour. At 65 miles per hour, it’s going to get roughly 15% worse gas mileage, or 21.25 miles per gallon. If you trim that back to 64 miles per hour, your gas mileage is a bit better – you’ll be getting 21.625 miles per gallon, more or less.

Now, let’s say you’re going on a 400 mile trip on the interstate and that gas is available for $3.25 a gallon.

If you go 65 miles per hour, it will take you 6 hours and 9 minutes to make the trip. You’ll burn through 18.82 gallons of gas, which will cost you $61.17.

If you go 64 miles per hour, it will take you 6 hours and 15 minutes to make the trip, six minutes longer. You’ll burn through 18.5 gallons of gas, which will cost you $60.13.

In short, driving one mile per hour slower will add six minutes to the trip and save you $1.04 in gas. Your savings simply by driving one mile per hour slower is $10.40 per hour. That, of course, is after-tax money.

That figure, as mentioned above, assumes a 25 mile per gallon car, but other mileages have similar savings. It also assumes that you’re slowing down a bit from a speed above 55 miles per hour.

So, should you just go 55 on any road you’re on? I wouldn’t do that. Instead, I’d stick to the posted speed limit and maybe go a mile an hour or two below that in the slower lane on an interstate.

Doing this serves three purposes. One, you’ll put cash in your pocket for the extra time you spent driving. Two, you’ll never get a speeding ticket. Three, you’re sticking more or less with the flow of traffic (going much slower would disrupt that), so you’re not disrupting traffic flow and endangering yourself that way.

The next time I’m rolling along some flat four lane road in southern Iowa, I’ll just set the cruise to a couple of miles per hour below the speed limit and roll along. Sure, I might get there five minutes later, but I know I won’t get pulled over for speeding, I’ve got something entertaining on the radio, and that bit of extra time will put a bit of money straight into my pocket.

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