So I know range drops noticeably when doing more than 70mph, is there a marked difference from 60-65 and 65-70?
In short, yes. Purely based on energy and resistance it’s clear the faster you go, the more energy you have to use.
Depending upon the car you have, its aerodynamics and its mechanical efficiencies you’ll see anything from a few miles more to tens of miles extra just by cutting your speed to 65 from 70. You’d get similar results by cutting again to 60 but there’s a compromise between efficiency and the time of your journey. For example, I got 243 miles out of my Ioniq 5 on a run with some in reserve and was driving at the relevant speed limit. I could have pushed that by going at 60 all the way but I’d have arrived late.
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Mark