- In DOE/ORNL model results for a midsize gasoline car, fuel economy is 45 MPG at 55 mph, slightly better than 43 MPG at 45 mph, then falls to 38 MPG at 65 mph and 32 MPG at 75 mph, a 28.9% drop from 55 to 75 mph.
- A midsize hybrid's fuel-economy edge over the gasoline car is 27.9% at 45 mph but exactly 0% at 65 mph; the diesel car keeps a real advantage (32.6% down to 15.6%) at every speed tested.
- At 15,000 highway miles per year and $3.10 per gallon, driving the gasoline car at 75 mph instead of 55 mph costs about $420 more per year in fuel.
01 Introduction
How fast you drive is one of the most controllable factors in fuel economy. The only speed-specific data in this site's source files is a DOE/ORNL model of a midsize conventional gasoline car, a midsize conventional diesel car and a midsize hybrid electric car at four steady speeds, so this article is limited to those results.
DOE Alternative Fuels Data Center, "Fuel Economy at Driving Speeds," sourced to Oak Ridge National Laboratory, Transportation Energy Data Book, Edition 39, Table 4.3302 Fuel Economy at Different Speeds
The table shows modeled fuel economy at four steady speeds for each of the three midsize cars.
| Speed | Gasoline (MPG) | Diesel (MPG) | Hybrid (MPG) |
|---|---|---|---|
| 45 mph | 43 | 57 | 55 |
| 55 mph | 45 | 55 | 46 |
| 65 mph | 38 | 45 | 38 |
| 75 mph | 32 | 37 | 33 |
The gasoline car's numbers show that fuel economy does not decline steadily from the moment you speed up: it is slightly better at 55 mph (45 MPG) than at 45 mph (43 MPG), then declines sharply, losing 28.9% from 55 to 75 mph. The diesel and hybrid cars are best at 45 mph, the lowest speed tested, and decline continuously from there.
Calculated: (32−45)÷45=-28.9% (gasoline, 55 to 75 mph). DOE Alternative Fuels Data Center, "Fuel Economy at Driving Speeds."Compared with the gasoline car, the hybrid and diesel results at each speed are:
| Speed | Hybrid vs Gasoline | Diesel vs Gasoline |
|---|---|---|
| 45 mph | +27.9% | +32.6% |
| 55 mph | +2.2% | +22.2% |
| 65 mph | +0.0% | +18.4% |
| 75 mph | +3.1% | +15.6% |
The data covers midsize cars only. This site has no comparable measured dataset for light trucks or SUVs at different speeds and does not publish a figure for them. For how fuel cost compares by vehicle type generally, see fuel cost by vehicle type.
Estimate your vehicle's driving cost using official EPA fuel economy data.
Use the Calculator03 Cost Impact of Speeding
The fuel cost of high-speed driving adds up over a year of commuting. Consider a driver who commutes 30 miles each way on the highway (60 miles/day, roughly 15,000 highway miles per year), using the gasoline-car MPG figures from the table above:
DOE Alternative Fuels Data Center, "Fuel Economy at Driving Speeds"; fuel cost calculated at EIA's 2025 annual average retail regular gasoline price of $3.10/gallon.| Speed | MPG | Annual Gallons | Annual Cost ($3.10/gal) | vs. 55 mph |
|---|---|---|---|---|
| 45 mph | 43 | 349 | $1,081 | +$48 |
| 55 mph | 45 | 333 | $1,033 | n/a |
| 65 mph | 38 | 395 | $1,224 | +$190 |
| 75 mph | 32 | 469 | $1,453 | +$420 |
Because 55 mph is the most fuel-efficient speed in this dataset for the gasoline car, it is also the cheapest: 45 mph and 65–75 mph all cost more per year. Driving at 75 mph instead of 55 mph costs this driver an additional $420 per year in fuel, while saving commute time. Whether that trade-off is worthwhile is a personal decision.
Cost comparison calculated from the table above.04 Data Sources
- DOE Alternative Fuels Data Center: "Fuel Economy at Driving Speeds," sourced to Oak Ridge National Laboratory, Transportation Energy Data Book, Edition 39, Table 4.33. afdc.energy.gov/data
- EIA: Annual average retail gasoline price, 2025. eia.gov