Dropping from 80mph to 70mph cuts motorway fuel use by roughly 10% to 15%, and where you fill matters even more. Motorway services average 171.4p a litre against 160.9p everywhere else - a 10.5p premium that adds £5.78 to a 55-litre fill. On a long trip, the refuelling decision often saves more than the driving does.
Speed is the dominant factor
Aerodynamic drag rises with the square of speed, so the power needed to push through the air climbs steeply. Below about 50mph, rolling resistance dominates; above it, drag takes over and every extra 10mph costs disproportionately.
| Speed | Typical mpg | Fuel cost | Time for 200 miles |
|---|---|---|---|
| 60 mph | 55 | £26.60 | 3h 20m |
| 70 mph | 48 | £30.50 | 2h 51m |
| 80 mph | 42 | £34.80 | 2h 30m |
The step from 70 to 80 costs about £4.30 and saves 21 minutes. Whether that is worth it is your call - but 80mph is above the limit, and the fuel penalty is real.
What else actually helps at speed
Take the roof box off
An empty roof box or roof bars can cost 10% to 20% at motorway speed. It is pure drag, and most people leave them on for weeks after the trip. This is the largest single avoidable penalty on the motorway.
Use cruise control on flat roads
Cruise control holds a steadier speed than most drivers manage, and small speed variations cost fuel. On genuinely hilly routes it can be counterproductive, since it will accelerate hard uphill where a driver would let speed bleed off.
Keep windows shut and use the air conditioning
The opposite of the town advice. At 70mph, open windows create more drag than the air conditioning compressor costs to run.
Leave a bigger gap
Sitting close to the vehicle ahead forces constant small throttle and brake adjustments. A larger gap lets you hold a steady speed and absorb changes in traffic without braking.
The refuelling decision usually beats the driving
All the driving technique on a 200-mile run might save you £4. Avoiding motorway services on a single 55-litre fill saves £5.78.
- Start full from a cheap local station. Supermarkets average 159.1p against 171.4p at services - 12.3p a litre.
- Plan a refuelling town and check prices before you go on our regional pages.
- If you must use services, add £10 to reach somewhere cheaper rather than filling.
- A short detour usually pays. At 40mpg a four-mile round trip costs about 72p in fuel against a potential £5.78 saving.
More detail in are motorway service station prices higher.
Fill before you join: the numbers
All the speed and smoothness advice above is worth a few percent. One motorway decision is worth double-digit pence: where you fill up. Motorway services average 171.4p for petrol against 160.9p at the other 7,745 forecourts we track - a 10.5p premium for the same fuel.
| Tank size | Extra at the 10.5p premium |
|---|---|
| 50 litres | £5.25 |
| 60 litres | £6.30 |
| 70 litres (van) | £7.35 |
The premium applies per stop, so it scales with the length of the trip: the transcontinental driver who fills at services twice is paying the tax twice. On a long journey with two services fills, that is a £10-15 tax on not planning - more than most drivers save in a month of careful accelerator work. Fill before you join, or plan a junction-town stop - a five-minute check of the towns along your route before a long journey costs less time than one services queue, and unlike the queue it pays for the trip, the full comparison, including when a detour off the motorway is and is not worth it, is in our motorway services pricing guide.
Lane discipline saves fuel too
Sitting in lane three and repeatedly closing on slower traffic means constant acceleration and braking. Settling into a realistic lane at a steady speed uses less fuel and is generally quicker over a long run than weaving between lanes.
Frequently asked questions
What speed is most efficient?
For most cars, somewhere between 45 and 60mph - below the motorway limit. On a motorway, 60 to 65mph is the practical efficiency sweet spot, though you need to consider traffic flow and journey time.
Does drafting behind a lorry save fuel?
It does reduce drag, and it is dangerous. You lose your view of the road ahead and your braking distance disappears. Do not do it.
Are roof bars really that bad?
Yes. Bars alone add measurable drag, and a box far more. If you use them occasionally, take them off in between - it is the easiest 10% available on motorway journeys.
Is it worth slowing down to avoid a fuel stop?
Sometimes. If reaching a cheaper station beyond your current range means easing off for twenty miles, the saving from avoiding a 10.5p premium on a full tank comfortably exceeds the fuel you save by driving slowly. Our fuel cost calculator will let you compare.
Is it worth filling only half a tank to save weight?
The maths says no. Twenty-five litres of fuel weighs about 19kg, and the economy gain from hauling 19kg less at a steady 70mph is a fraction of one percent - pence per tank, not pounds. Fill properly at a cheap forecourt instead; that single decision is worth about 10p a litre.
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