<< Driving for 5 miles at 55mph takes just over a minute longer than it would at 70mph. >>
And presumably uses quite a bit less fuel ?
Absolutely, but I didn't mention it as I assumed the OP was more concerned about the time lost.
Having been in similar situations while driving a bus, that is to say being held up by slower traffic (invariably when already late!), I know it feels like the time lost is huge, but in reality was rarely more than a few minutes, even if stuck behind a tractor!
But going back to the efficiency, the AA reckon an average car will be 9% less efficient at 70mph than at 60mph and 15% less than at 50mph, so around 12-13% difference at 55mph sounds about right. They (the AA) also reckon that driving at 80mph results in 25% less efficiency than at 70mph!
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