Is It Worth Driving Slower on the Motorway?
Most UK motorway drivers sit somewhere between 70 and 85 mph. Some drive at the legal limit, some go faster. The conventional wisdom is that driving faster costs more in fuel — but how much more, exactly? And is the time you save actually worth the extra cost?
We've crunched the numbers using real fuel efficiency data for a typical UK family car (40 MPG rated, petrol, at current pump prices of 156.8p/litre).
The numbers — 60, 70, and 80 mph compared
Let's take three common motorway journeys and compare the cost and time at different speeds:
London to Manchester (208 miles, mostly motorway)
| Speed | Journey time | Fuel cost | CO2 emitted |
|---|---|---|---|
| 60 mph | 3h 28m | £18.20 | 37.0 kg |
| 70 mph | 2h 58m | £19.10 | 38.9 kg |
| 80 mph | 2h 36m | £21.50 | 43.8 kg |
| 90 mph | 2h 19m | £24.30 | 49.5 kg |
London to Edinburgh (414 miles, mostly motorway)
| Speed | Journey time | Fuel cost | CO2 emitted |
|---|---|---|---|
| 60 mph | 6h 54m | £36.20 | 73.6 kg |
| 70 mph | 5h 54m | £38.00 | 77.3 kg |
| 80 mph | 5h 10m | £42.80 | 87.1 kg |
| 90 mph | 4h 36m | £48.30 | 98.3 kg |
Key finding: Driving at 80mph instead of 70mph on the London to Edinburgh journey costs £4.80 more in fuel and produces nearly 10kg more CO2 — but only saves 44 minutes. That's roughly £6.50 per hour of time "saved" in fuel costs alone — before factoring in the speeding risk.
Why does speed affect fuel consumption so much?
The physics is straightforward — aerodynamic drag increases with the square of speed. That means doubling your speed requires four times as much force to overcome air resistance. At 80mph, your car is pushing through 31% more air resistance than at 70mph.
Most petrol cars hit peak fuel efficiency around 50-55mph. Above that, efficiency slowly declines as speed increases. The relationship accelerates sharply above 70mph — which is why the gap between 70 and 80mph is larger than the gap between 60 and 70mph.
The "is it worth it?" calculation
The real question isn't just "does it cost more?" — it's "is the time saving worth the money?" Here's how it breaks down for a regular motorway commuter doing 200 miles per week:
| vs driving at 70mph | 80mph | 60mph |
|---|---|---|
| Weekly fuel cost difference | +£2.30 more | −£0.85 less |
| Annual fuel cost difference | +£120 more | −£44 less |
| Weekly time difference | −14 min faster | +17 min slower |
| Annual CO2 difference | +47 kg more | −19 kg less |
💡 The verdict
80mph vs 70mph: You save 14 minutes per 200-mile journey but spend £120 more per year in fuel. That works out to £8.57 per hour of time "saved" — before accounting for the increased accident risk and potential fine. For most people, not worth it.
60mph vs 70mph: You spend 17 extra minutes on a 200-mile journey but save £44/year in fuel and reduce CO2 by 19kg. A reasonable trade-off on longer journeys where you're not in a rush.
What about the time saving in real life?
It's worth noting that the time savings from driving faster are often less than they appear. On a typical motorway journey, you'll spend time in traffic, at roundabouts, and in service stations regardless of your motorway speed. A 14-minute saving on the open motorway stretch might translate to 8-10 minutes of real-world difference in actual arrival time.
And if you're stopped for speeding — a 3-point, £100 fixed penalty is the minimum. That's the equivalent of several months of fuel savings from driving at 80mph wiped out in one stop.
The environmental case for slowing down
Beyond money, the CO2 difference is significant at scale. If every UK driver doing regular motorway journeys reduced their speed from 80mph to 70mph, the collective fuel saving would be enormous. On an individual level, the 47kg annual CO2 saving from driving at 70mph instead of 80mph (for a 200-mile/week commuter) is equivalent to the carbon absorbed by two trees per year.
The smart compromise: Most drivers find that sitting at 65-68mph on the motorway is the sweet spot — faster than many lorries, clearly within the speed limit, and noticeably more efficient than 75-80mph. Set your cruise control and relax.
Use our calculator to see the exact numbers for your car, your route, and your typical driving speed — including the three-way comparison that shows what you'd save at the speed limit.