Doing the same motorway run but stopping for fuel sooner than usual? The biggest gains often come from cutting drag and rolling resistance, not finding a different road. On a long, steady drive, small inefficiencies add up quickly because the car is working against air and tyres for miles at a time.

Drag matters most

At motorway speed, air resistance is usually the main fuel penalty. That is why a small drop in cruising speed can make a bigger difference than most drivers expect, especially in a petrol car or a loaded SUV. Cruise control helps on flatter sections, but holding an aggressive speed into long inclines can push consumption up sharply.

Extra kit on the outside matters too. Empty roof bars can knock a little off motorway economy all the time, while a roof box can raise fuel use by roughly 10–25% once speeds climb. A rear bike rack is often better than a roof-mounted one, and simply removing unused bars after a trip is one of the easiest wins.

Quick five-minute checks

That is the easy part. Less obvious is when the car itself is wasting fuel, and a few quick checks can catch the usual causes before they become a bigger bill.

  • Check tyre pressures cold, ideally once a month and before a long run. Even slightly low pressures increase rolling resistance for the whole journey.
  • Look at the tyres while you are there. Heavy wear on one shoulder or a saw-tooth feel across the tread can point to poor alignment.
  • Remove drag you have forgotten about: roof bars, a roof box, a bike rack or heavy kit that lives in the boot permanently.
  • Pay attention to symptoms after driving. If the car feels held back, pulls to one side or one wheel seems much hotter than the others, a brake may be dragging.

When DIY stops

If economy has dropped for more than a tank or two and the simple checks do not explain it, a workshop inspection is usually worth it. A wheel alignment check often takes around 30–60 minutes and is usually one of the lower-cost jobs. Brake-drag diagnosis can take a similar time, but the bill rises if a caliper, slider or hose is sticking. An overdue service can also show up on the motorway, particularly if the air filter is dirty or the engine is not running cleanly, though modern fault finding may need diagnostic equipment.

For most drivers, the best routine is simple: check pressures regularly, strip off unused roof kit, and investigate any sudden drop in motorway mpg rather than driving around it. On long journeys, those small fixes are usually where the real savings are.