The average car consumes 38 per cent more fuel at 70mph than it does over the same distance at 50mph. At 60mph it uses 34 per cent more than at 40mph. [And at 100 mph, you use 5 times as much as fuel as at 50 mph!]
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My maxima gets better mileage going 70 than it does going 55.
You’ve driven a trip on the highway at 55 mph for comparison? Or are you just using bogus math and comparing apples to oranges, with a 70 mph trip on the highway versus a 55 mph trip on a slower road?