It's a bit of a pain that economy nosedives as we go faster, since motorways are intended to get us to our destinations quickly.
A steady cruise is also the most efficient way to drive. However, pootling along at 30mph would be a drag and probably not very safe.
You might find yourself in trouble with the law, too. In January, a 58-year-old woman was banned from driving for a week after being caught driving at just 10mph along the M32.
Still, the motorway is where the biggest savings are to be made. Realistically, in good weather and traffic conditions, drivers do over 60mph. It's how much faster you go that matters to your pocket.
Take a small family car like the Nissan Qashqai. At 60mph you'll be doing 39mpg, yet at 90mph you'll be doing about half that.
Yes, you'll get where you're going a bit quicker, but you'll be throwing away an extra 11p every mile. That's 10 on a 90-mile journey. Do that a couple of times a week and it could set you back an extra 1000 a year.
It's not only money you'd save. The UK Energy Research Centre estimates that proper enforcement of the 70mph limit would slash CO2 emissions by 3.7 million tonnes.
Lowering the limit to 60mph would cut CO2 by a further 3.5 million tonnes. That's the equivalent of taking three million Ford Focuses off the road.