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Wouldnt that be a factor of the gearing? Highest gear at minimum revs is 56mph?
Wouldnt 40 be a gear lower at higher revs?
In an EV with a single reduction gear it must surely be more of a linear efficiency/speed equation?
Depends entirely on the car. Pretty much every ICE car I've driven would be happy in top at well under 56mph, even a leggy Jag XF 3.0 diesel with an eight-speed box will change up into top at around 50mph.

ICE engines' efficiency is represented by a diagram called a BSFC map (brake specific fuel consumption), that shows the fuel efficiency in g/kWh (grammes of fuel per kilowatt-hour of output power) for a given engine speed (rpm) and load. See the BSFC article on Wikipedia — the map shown as an example on that page shows a peak efficiency of 206g/kWh at about 2200rpm and ~85% load.

Setups like diesel electric trains will run the big diesel engine as close to the efficiency peak as they can, and I'd assume that REX cars will do likewise.

It also shows that lower engine speeds aren't always better: at high loads, allowing the engine to "lug" could be worse than changing down a gear.

To achieve maximum possible published efficiency at 56mph, you'd size the engine and specify the gearbox to land on that efficiency peak at that speed. However, if you had a car that was at 85% throttle at 56mph and 2200rpm, it wouldn't be much fun to drive (old non-turbo Escort diesel?)! In reality, you'd work out your gearing to make the best of the map for the engine you're using, perhaps on the 225g/kWh contour at 7 bar and 1500rpm with a more powerful engine.

But you don't directly care about g/kWh, you care about mpg (or p/mile!). Ignoring everything except aero drag, it takes 1.96× as much power (kW) to push the car through the air at 56mph than at 40mph (56²÷40²). You take about 1.4× longer to travel that distance at 40mph, so overall need 1.4× the energy to cover a given distance at the higher speed. Yes, the BSFC map does show reduced thermodynamic efficiency at lower loads — but not a factor of 1.4× in that region (225g × 1.4 = 315g), so 40mph will use less fuel.

As far as the EV motor equivalent to the BSFC map goes, I don't know what it would look like, but I'd imagine it'd be much less variable than the ICE map.
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