You’ll use noticeably more fuel at 70 mph than at 50 mph, and the commonly quoted driving-theory-test figure is “about 15% more fuel” for 70 vs 50 over the same distance.

Below is a clear, rounded picture using public guidance and test data.

Quick Scoop: Short Answer

  • UK-style theory question answer:
    • Driving at 70 mph uses about 15% more fuel than driving at 50 mph for the same distance.
  • Other motoring and efficiency sources suggest “up to around 30% more” can be used at 70 than 50, depending on the car and conditions.
  • Real-world lab data shows that going from 50 to 70 mph typically cuts fuel economy by roughly 25–30% , meaning you burn roughly 25–30% more fuel per mile.

So a realistic band is:

Rough rule of thumb: 15–30% more fuel at 70 than at 50 per mile.

Why higher speed burns more fuel

  • Above about 40–50 mph, aerodynamic drag ramps up quickly, and the power needed to push air out of the way grows with roughly the square of your speed.
  • That extra power demand at 70 mph forces the engine to use more fuel every second, and you’re also covering more miles per hour, so fuel burn per mile rises.

A U.S. Department of Energy–backed analysis of dozens of vehicles found:

  • The best fuel economy is typically between 40 and 50 mph in top gear.
  • From 60 to 70 mph , average fuel economy drops a further 14% , and from 50 to 60 mph it drops about 12.4%.

Putting that together, many cars lose on the order of 25–30% fuel economy going from 50 to 70 mph, which is the same as needing 25–30% more fuel per mile.

Simple example with numbers

Imagine your car does:

  • 50 mpg at 50 mph
  • A 25% drop moving to 70 mph means fuel economy at 70 mph is:
    • 50 mpg×(1−0.25)=37.5 mpg50\text{ mpg}\times (1-0.25)=37.5\text{ mpg}50 mpg×(1−0.25)=37.5 mpg

To go 100 miles:

  • At 50 mph: 100 ÷ 50 = 2.0 gallons
  • At 70 mph: 100 ÷ 37.5 ≈ 2.67 gallons

That’s roughly 33% more fuel for the same trip at 70 vs 50 in this example.

Theory test vs real world

You’ll see two kinds of answers floating around:

  • Driving theory / exam style answer:
    • “About 15% more fuel at 70 vs 50.”
  • Broader ‘up to’ guidance:
    • “Up to around 30% more fuel ,” which appears in forum and Q&A style references linked to DVLA-type advice.
  • Engineering and lab-based data:
    • DOE-backed and similar analyses show roughly 25–30% more fuel per mile at 70 vs 50 for many modern cars.

These aren’t contradictions so much as:

  • 15%: a neat, memorable exam fact.
  • 25–30%: what a lot of cars actually do in steady-speed testing on real roads or dynamometers.

What affects your exact number?

The precise % for your car depends on:

  • Shape and size (aerodynamics and frontal area).
  • Engine type and gearing (where the engine sits on its efficiency curve at 50 vs 70).
  • Tyres and pressure, plus roof racks or boxes adding drag.
  • Wind, gradients, and how smoothly you drive (hard acceleration, tailgating, etc.).

But for a normal modern petrol or diesel car on a motorway , it’s very reasonable to plan around:

About a quarter more fuel at 70 than at 50 for the same journey, with theory-test answers often quoting ~15%.

Information gathered from public forums or data available on the internet and portrayed here.