MilesheetRoof boxes, bike racks and towing: what they really cost
The bolt-ons that quietly ruin your economy, ranked by how much they cost and how easily you can stop paying for them.
Cars are shaped the way they are for a reason, and most of that reason is air. Anything bolted to the outside interferes with the shape, and the bill arrives at every fill-up.
The rough ranking
From most expensive to least, at motorway speeds:
- Towing a caravan — economy can halve. There is no trick here: you are pushing a second box through the air and hauling its weight.
- A loaded roof box — commonly 10–20% at 70 mph, more on a small efficient car where the box is large relative to the vehicle.
- An empty roof box — nearly as bad as a full one. The drag is the shape, not the contents.
- Bare roof bars — often 5–10%. Less than a box, absolutely not nothing.
- A bike rack on the towbar — usually gentler than a roof rack, because it sits in air the car has already disturbed.
- Windows down at speed — measurable, though generally less than the air conditioning it is replacing.
The one that actually costs people money
Bars and boxes left on all year.
The trip they were fitted for lasted a fortnight. The 8% they cost carries on until somebody gets a spanner out. Over a year of ordinary driving, that is a real number: on 10,000 miles at 45 mpg and 145p a litre, roughly £1,460 of fuel, so 8% is around £117 for carrying an empty box you are not using.
Ten minutes with a spanner is one of the better hourly rates available.
Towing: what actually helps
You cannot make a caravan aerodynamic, but you can avoid making it worse.
- Load it nose-heavy within the noseweight limit — stability first, but a stable outfit also needs fewer corrections
- Keep tyre pressures right on both vehicle and trailer
- Slow down. Drag rises with the square of speed and you are now presenting a great deal more frontal area to it
- Take the roof box off the car if the caravan is carrying the luggage
Measure your own
Every figure above is a typical range. Your car, your box, your speed and your roads will produce your own number, and the only way to find it is to compare tanks.
Log a few fill-ups with the box on and a few with it off, and Milesheet will show the difference as a change in your real MPG between full tanks. It is a satisfying way to settle an argument, and occasionally a surprising one.