Speed and Fuel Economy: What Happens Above 50 MPH
- Above ~50 mph, fuel economy generally drops as speed increases (DOE).
- Use cost per mile to see budget impact from a small MPG change.
- Small changes matter most if you drive a lot of highway miles.
What We Know (Sourced)
DOE’s fuel-saving guidance highlights a consistent pattern: fuel economy usually declines as speeds rise. In its “Driving More Efficiently” guidance, DOE notes that gas mileage typically decreases rapidly at speeds above 50 mph and provides an illustrative way to think about the cost impact of driving faster. DOE also states that speeding can reduce fuel economy by a percentage range. See: DOE — Driving More Efficiently.
The Alternative Fuels Data Center (AFDC) also summarizes behavioral techniques that conserve fuel, including driving smoothly and avoiding aggressive driving. See: AFDC — Efficient Driving to Conserve Fuel.
Why Speed Affects MPG
At higher speeds, vehicles generally require more power to maintain speed. That increased demand can raise fuel use per mile. The exact “best” speed varies by vehicle and conditions, but the directional effect (higher speed → more fuel per mile) is the reason DOE emphasizes moderating speed in its fuel-economy guidance.
This is also why it’s helpful to think in fuel consumption terms (gallons used per fixed distance) rather than only MPG. EPA includes a fuel consumption rate (gallons per 100 miles) on gasoline labels and explains it relates directly to fuel expenditures. See: Gallons per 100 miles explained.
How to Estimate the Cost of Speeding
You don’t need perfect data to estimate the budget impact of a speed-related MPG change. You just need:
- Your approximate highway miles (per week, month, or year)
- Your MPG at a “normal” speed vs a higher speed (even a rough estimate)
- Your average gas price
If you want to do this quickly, use our Fuel Cost Calculator. For day-to-day budgeting, the Commute Gas Cost Calculator is often even faster.
What to Do Next
If you’re trying to improve highway fuel economy without changing vehicles:
- Pick a consistent cruise speed and avoid unnecessary speed swings.
- Drive smoothly (AFDC summarizes efficient-driving behavior techniques).
- Keep tires properly inflated (see: tire pressure and fuel economy).
If your fuel economy has dropped suddenly, also check for mechanical causes: Why is my car getting bad gas mileage?
Why It Matters
If you drive many highway miles, a modest MPG change can translate into noticeable gallons used over a year. Converting that into cost per mile helps you see whether a change is worth the tradeoff. If you’re comparing cars, use EPA label consumption (gallons/100 miles) and annual fuel cost estimates as a baseline (see: EPA label basics).
See Your Cost per Mile
Enter your MPG and gas price to compute fuel cost per mile and estimate annual spending.
Use the Cost Per Mile CalculatorFrequently Asked Questions
Does speed always reduce MPG?
In general, higher speeds can reduce fuel economy because the vehicle needs more power to maintain speed. DOE notes that gas mileage typically decreases rapidly above 50 mph and provides guidance on how speeding affects fuel economy.
Is Combined MPG on the EPA label a good predictor for freeway commuting?
Combined MPG is a standardized mix. For mostly-freeway commuting, Highway MPG may be closer—but real results still depend on speed, traffic, weather, and vehicle load.
How can I measure the MPG difference between two cruise speeds?
The safest approach is to track fuel economy over multiple similar trips at each speed (same route, similar weather, similar load) and compare averages. Use fuel cost formulas to translate the MPG difference into dollars.
What’s the easiest way to estimate the cost difference?
Use Fuel Cost = (Miles ÷ MPG) × Price per Gallon. Run it twice (two MPG values) and compare. Our Fuel Cost Calculator does this quickly.