Understanding the Charts

Cost/Gallon: When driving 50 mph, price per gallon is assumed to be the same as the pump price. At higher speeds, the estimated price per gallon is increased based on the additional fuel you use by driving faster. The price of fuel doesn't actually go up, but this is a useful way to put the cost of driving faster in context.

Cost/Gallon = Fuel Price at Pump × Est. MPG at Higher Speed

÷ Est. MPG at 50 MPH

Cost/100 Miles: This is useful for estimating fuel costs for long trips.

Cost per 100 Miles = Fuel Price ÷ Estimated MPG × 100

Save Money: This graph shows how much money you can save by slowing down. Cost per gallon and cost per 100 miles show the difference in cost at your typical highway speed and the cost at the reduced speed.

Savings per extra time incurred is calculated as the money saved by slowing down divided by the extra time incurred:

Savings = (Fuel Price * (1 ÷ MPG T )-(1 ÷ MPG R )) ÷ ((1 ÷ Speed T )-(1 ÷ Speed R ))

where

MPG T = fuel economy at typical highway speed

= fuel economy at typical highway speed MPG R = fuel economy at reduced highway speed

= fuel economy at reduced highway speed Speed T = typical highway speed

= typical highway speed Speed R = reduced highway speed

The Study

Speed-mpg relationships are based on dynamometer tests of 74 light-duty vehicles representing a variety of manufacturers, nine vehicle classes, engine sizes 1.5 L to 6.2 L, and model years 2003 to 2012. Almost all are conventional gasoline-powered vehicles. Only one diesel and two hybrids are included; no plug-in hybrids or electric vehicles were tested.

These estimates work best for well-maintained modern gasoline light-duty vehicles and will likely be less accurate for diesels, hybrids, and vehicles with cylinder deactivation. Their applicability to plug-in hybrids and electric vehicles is unknown.

We estimate vehicle MPG at steady-state cruising speeds based on the vehicle's EPA "highway" fuel economy rating. These estimates are typically higher than both the EPA highway MPG ratings and the MPG consumers experience in on-road highway driving. Therefore, our cost and savings estimates should be conservative.