How is it much more fuel efficient to travel at say 50 mph for longer than 70 mph for a shorter period of time. Like if I was going a set distance, if I drive slower I’m using the car longer so why isn’t it the same fuel being used.
In: 15
Almost universally down to the engine speed.
In top gear, with the engine barely rotating, you’ll do 50mph on a flat road. The engine speed will barely be above idle (that speed necessary to keep it rotating), often only around `1000RPM
To get to 70mph, you would have to rotate the engine at a few thousand more RPM.
If you provided another gear, you would be able to do 70mph at engine idle, but at greater engineering cost, and with air resistance, etc. it would actually not be as simple as “yet another gear range”, you would have to do the maths to make it as efficient.
You then risk getting into a situation like trucks have where you have to have LOTS of gears (and gear changes) to make it as efficient at a speed that you won’t necessarily hold for a long time (e.g. every time you want to change lanes or slow slightly, you’d have to change down a gear… generally you can just stay in top gear on a motorway and concentrate purely on driving).
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