How much fuel economy drops with speed
The model’s gasoline car gets its best fuel economy at 55 mph, so slowing below that doesn’t keep saving: at 45 mph it uses a little more gas than at 55. The savings come from coming down from highway speeds, not from crawling.
The Department of Energy’s Alternative Fuels Data Center publishes modeled fuel economy at four steady speeds for three midsize cars. The gasoline car’s figures, applied to a 500-mile drive:
| Speed | Modeled MPG | Gallons | Gas | Driving time |
|---|---|---|---|---|
| 45 mph | 43 | 11.63 | $52.07 | 11 h 6.7 min |
| 55 mph | 45 | 11.11 | $49.76 | 9 h 5.5 min |
| 65 mph | 38 | 13.16 | $58.92 | 7 h 41.5 min |
| 75 mph | 32 | 15.63 | $69.97 | 6 h 40 min |
MPG figures are modeled for one midsize conventional gasoline car (Oak Ridge National Laboratory, Transportation Energy Data Book #39, Table 4.33, from Argonne’s Autonomie model). The same chart shows a hybrid losing efficiency more evenly between 45 and 75 mph. Your car will differ.
Driving time is miles ÷ speed at a constant speed, with no stops, to a tenth of a minute. Gas is miles ÷ MPG × price from the trip gas cost engine, using unrounded gallons.
Reference: DOE Alternative Fuels Data Center: Fuel economy at various driving speeds · EIA: Gasoline and Diesel Fuel Update
What the time saved costs in gas
Driving faster buys time with gas. Dividing the extra gas by the hours saved puts a price on each hour:
| Speed change | Extra gas | Time saved | Extra gas per hour saved |
|---|---|---|---|
| 55 → 65 mph | $9.16 | 84 min | $6.54 |
| 65 → 75 mph | $11.05 | 61.5 min | $10.78 |
| 55 → 75 mph | $20.21 | 145.5 min | $8.33 |
Worked from the rounded figures in the table above: extra gas ÷ (minutes saved ÷ 60). At a higher gas price, every amount rises in proportion.
Estimate it for your own car
You can’t look up your car’s MPG at 75 mph, but you can bracket it. FuelEconomy.gov estimates that slowing down 5 to 10 mph improves fuel economy by 7% to 14% on average.
Run the trip calculator twice: once with your usual MPG, once with a lower MPG for the faster pace. For example, 1,000 miles at 30 MPG costs $149.27 at $4.478 a gallon; at 27 MPG, 10% lower, it costs $165.85, a difference of $16.58.
Your trip computer’s average MPG over a long highway stretch is the best number to start from.
Compare two MPG figures for your trip ↗
Reference: FuelEconomy.gov: Driving more efficiently
Other things that cost gas on a road trip
FuelEconomy.gov lists these effects for typical vehicles:
- Aggressive driving (speeding, rapid acceleration and braking) can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic.
- A large, blunt roof-top cargo box can reduce fuel economy by around 10% to 25% at Interstate speeds (65 to 75 mph); rear-mount boxes or trays reduce it by much less, 1% to 5% on the highway.
- An extra 100 pounds in the vehicle could reduce MPG by about 1%, more in smaller vehicles.
- Idling can use a quarter to a half gallon of fuel an hour, depending on engine size and air conditioner use.
- Cruise control helps hold a constant speed on the highway and, in most cases, saves gas.
Reference: FuelEconomy.gov: Driving more efficiently
Drive at a speed that is legal and safe
Speed limits and road conditions come first; the savings here never justify driving slower than traffic safely allows. FuelEconomy.gov notes that observing the speed limit is also safer.
Reference: FuelEconomy.gov: Driving more efficiently
Use the calculator with your numbers
The worked example below prices the same 500 miles at the model’s 65 mph and 75 mph fuel economy, using the trip gas cost calculator’s engine.
| Item | Example result |
|---|---|
| Gas at 65 mph | $58.92 |
| Gas at 75 mph | $69.97 |
| Extra gas for driving 75 | $11.05 |
| Time saved | 62 minutes |
MPG by speed is modeled for one midsize gasoline car; use your own car’s MPG in the calculator.
Illustrative figures, not market averages. The underlying costs are calculated with the same engine as the tool; additional derived measures are labeled.
Price my trip at two speeds ↗About this guide
Prepared by RealCostTools with AI-assisted research and drafting. Sources and calculations were checked September 27, 2026. MPG by speed is a DOE model of one car type; gas uses EIA’s U.S. average for the week of September 21, 2026.
Sources and calculation notes
- DOE Alternative Fuels Data Center: Fuel economy at various driving speeds
Midsize conventional gasoline car: 43 MPG at 45 mph, 45 at 55, 38 at 65, 32 at 75 (Argonne Autonomie model; Oak Ridge National Laboratory, Transportation Energy Data Book #39, Table 4.33; worksheet last updated May 2021). Checked September 27, 2026.
- FuelEconomy.gov: Driving more efficiently
Checked September 27, 2026: gas mileage usually decreases rapidly above 50 mph; slowing down 5 to 10 mph gives a 7% to 14% fuel economy benefit on average; aggressive driving, roof cargo, extra weight and idling figures as quoted. Estimates for typical vehicles.
- EIA: Gasoline and Diesel Fuel Update
U.S. regular gasoline, $4.478 a gallon for the week of September 21, 2026, released September 22, 2026; all taxes included. Checked September 27, 2026.
- RealCostTools: trip gas cost methodology
The formulas behind the gas amounts in this guide.
Read the trip gas cost methodology for the model assumptions and excluded costs.
Published . Sources checked . Editorial policy · Corrections