The most fuel efficient way to travel on motorways is to follow lorry and set cruise control. It will mostly travel at 65 MPH and no need for sudden acceleration and braking while trying to overtake other vehicles.

If I drive at 35 MPH on motorway I shall die out of boredom.

Aerodynamic drag squares as speed doubles, so if your car produces 10 units of resistance at 35mph, it won’t produce 20 units at 70mph but 100 resistance units. Going quickly is pretty labour intensive.

That arithmetic isn't correct - 10 units of aerodynamic resistance at 35 mph will become 40 units at 70 mph - it's the speed increase squared.

Above a certain speed, however, rolling and wind resistance have an increasing counter-effect. Plus the fact that the optimum energy is extracted from the fuel near the top of the engine's torque curve, which for a typical petrol engine is about 3000rpm, or 2000 for a diesel.

Wind resistance squaring is a law of physics; unavoidable.

Below about 50 other variables like torque, fuel type and gearing can equal the physics. Above that no.

We tow a 1200kg caravan with a HDi 1.6/115PS Berlingo. The 'van has a mildly aerodynamic front but a bluff vertical rear demonstrates this amply; there's a reason lots of well designed cars (and trains) have streamlined rears. At 50 in still air we might just get 35mpg. Push it in a French motorway where, with Gross Train Weight under 3.5 tonnes, we're subject the normal 110/130 at 110 and we're in the low twenties mpg wise.

Winds are like on a push bike; usually an againsterly.

My 1.2 petrol Fabia's consumption figure scooted up smartly in the long 50 limits for M/way upgrades. Much less so since the limit was raised to 60.

Edited by Bromptonaut on 01/09/2022 at 23:45

My 1.2 petrol Fabia's consumption figure scooted up smartly in the long 50 limits for M/way upgrades. Much less so since the limit was raised to 60.

Back about 10 years 14 miles of the wifes 20 mile each way commute was on the motorway at 70 mph (she went before it got busy and came home mid afternoon before it was again busy). Then there was 2 years of works with a 50 mph limit to turn it into "smart motorway". The mpg of the diesel Focus went up 10% and considering only about 1/2 of her mileage was the commute it showed how dropping 20 mph really helped save fuel.

But she was bored to death and as soon as the works were complete back to 70 mph but since she retired soon after no idea if the mpg went back down.

Edited by thunderbird on 02/09/2022 at 08:00