Where do I find my baseline MPG at 60 mph?

Most trip computers let you reset average economy on a motorway cruise at steady 60 mph. Alternatively, manufacturer specifications sometimes list economy at constant speeds, or you can use a fuel-log average from a long motorway journey.

What is a typical percentage drop per 10 mph?

For many passenger cars, efficiency falls 10–15 % per 10 mph above 60 mph due to quadratic drag increases. Heavier or less aerodynamic vehicles (SUVs, vans) often see 15–20 % drops, while streamlined hybrids may lose only 8–12 %.

Can I use this calculator for speeds below 60 mph?

Yes. The tool treats each 10 mph reduction as a negative drop, effectively a percentage gain. However, real-world city driving includes stop-start losses that offset the aerodynamic benefit of lower speeds.

Why does the calculator clamp the adjustment between –50 % and +80 %?

Extreme percentage inputs can produce impossible results (negative or infinitely high MPG). The clamps keep outputs within physically plausible bounds, though the model remains an approximation outside moderate speed ranges.

Does this account for wind or hills?

No. The calculation assumes flat terrain and no headwind or tailwind. A strong headwind at 70 mph can add the aerodynamic resistance of 80+ mph, significantly reducing MPG beyond the percentage drop you enter.

Fuel & Efficiency

Speed vs Fuel Consumption

Calculate how highway speed affects your fuel economy using a percentage-drop model.

Last updated: 25 June 2026

What this tool does

This calculator estimates fuel economy at a target speed by applying a linear percentage drop (or gain) for each 10 mph deviation from a 60 mph baseline. You enter your vehicle's MPG at 60 mph, the percentage efficiency loss per additional 10 mph, and your target speed; the tool computes adjusted MPG. The model assumes a constant percentage rate and clamps the total change between –50 % and +80 % to prevent unrealistic outputs.

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Formula

MPG_{target} = MPG_{60} \times (1 - \frac{d \times ( \frac{v - 60}{10} )}{100})

MPG_{target}

MPG at target speed

MPG_{60}

Baseline MPG at 60 mph

v

Target speed in mph

d

MPG drop per 10 mph rise (%)

How Speed vs Fuel Consumption works

Aerodynamic drag rises roughly with the square of velocity, so fuel economy typically peaks between 50 and 65 mph and falls at higher speeds. This calculator applies a user-defined percentage drop (or gain) for every 10 mph step away from 60 mph. If you drive slower than 60 mph, the tool treats each 10 mph reduction as a negative drop—effectively an efficiency gain. The output shows predicted MPG at your chosen speed and the absolute change from the baseline.

The formula

MPG_target = MPG₆₀ × (1 − [ΔSpeed ÷ 10 × drop% ] ÷ 100)
where ΔSpeed = target speed − 60. The total percentage adjustment is clamped between –50 % and +80 % to keep results within physically plausible bounds.

Where this method is most accurate

The linear percentage model works best for moderate speed ranges (50–80 mph) and modern vehicles with consistent gearing. Real-world fuel consumption curves are non-linear—losses accelerate above 70 mph and gains taper below 50 mph—so large deviations from 60 mph may over- or under-estimate actual economy. The calculator also assumes flat terrain, no wind, and steady cruising; acceleration, braking, and traffic reduce real MPG.

What this tool does not do

It does not measure your vehicle's actual MPG at 60 mph; that baseline must come from your trip computer, fuel logs, or manufacturer data. It does not account for vehicle-specific factors like transmission efficiency, tyre pressure, or payload. The percentage-drop rate is user-supplied, not derived from your car's aerodynamic profile, so accuracy depends entirely on the quality of that input.

Disclaimer

This tool is for educational comparison only. It produces estimates based on user-entered parameters and does not constitute vehicle performance advice, emissions compliance guidance, or a guarantee of on-road fuel economy. Actual consumption varies with driving style, weather, load, and mechanical condition.

Questions

Where do I find my baseline MPG at 60 mph?: Most trip computers let you reset average economy on a motorway cruise at steady 60 mph. Alternatively, manufacturer specifications sometimes list economy at constant speeds, or you can use a fuel-log average from a long motorway journey.
What is a typical percentage drop per 10 mph?: For many passenger cars, efficiency falls 10–15 % per 10 mph above 60 mph due to quadratic drag increases. Heavier or less aerodynamic vehicles (SUVs, vans) often see 15–20 % drops, while streamlined hybrids may lose only 8–12 %.
Can I use this calculator for speeds below 60 mph?: Yes. The tool treats each 10 mph reduction as a negative drop, effectively a percentage gain. However, real-world city driving includes stop-start losses that offset the aerodynamic benefit of lower speeds.
Why does the calculator clamp the adjustment between –50 % and +80 %?: Extreme percentage inputs can produce impossible results (negative or infinitely high MPG). The clamps keep outputs within physically plausible bounds, though the model remains an approximation outside moderate speed ranges.
Does this account for wind or hills?: No. The calculation assumes flat terrain and no headwind or tailwind. A strong headwind at 70 mph can add the aerodynamic resistance of 80+ mph, significantly reducing MPG beyond the percentage drop you enter.

Spotted something off?

Calculations or display — let us know.

Sources & Methodology

The calculator multiplies baseline MPG at 60 mph by (1 − percentage_change ÷ 100), where percentage_change = (target_speed − 60) ÷ 10 × drop_rate_per_10mph. The total adjustment is clamped between –50 % and +80 %. This linear-percentage model approximates the effect of aerodynamic drag, which increases non-linearly with speed but can be linearised over narrow ranges for estimation purposes.

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