With a few exceptions there are generally no ‘shiftable’ gears, attached to an EV motor, therefore the rpm is more or less related to speed.
Even if one knew the most economic region for a MG5 motor, keeping it at that speed would another story so while I’m a believer of the ‘small wheel paradox’ I fail to understand why the motor efficiency effect
I might not have explained myself very well so hopefully this will help. You have raised good points re keeping in the zone and not having a traditional multispeed gearbox so have included those in the explanation.
This is a typical AC motor efficiency graph and around the 50nm/2000 rpm is an interesting area where small rpm changes can change the efficiency. No gears well actually it is a single speed reduction box - typcally around 10::1. The ratio varies. This means that with a bigger wheel that does not need to turn as much to cover the same distance, the wheel rpm is reduced. Multiply that by the gear ratio by 10 and the motor RPM can very significantly. A 20rpm reduction in a wheel speed is a 200rpm reduction in motor speed.
It gets worse because at a slower RPM you would need more torque and so the efficiency point can move in both directions. The gear ratio is designed to keep in the sweet spot as best it can and that is dependent on the wheel size and gear ratio. Change the wheel size and the operational point moves. Dependent on the motor that will typically make things worse. A 1% change is neither here or there but a 5% or greater change is a big factor. So when designing a powertrain, the design has to work out what is needed to run the motor at it's most efficient for most of the time. That might vary with a city car compared to hot hatch version. Going from a 22 inch diameter wheel to 24inch with a 10:1 reduction is a 1000 rpm motor speedreduction - that can be a big change. The graph shows that at 50nm going from 2000 rpm to 1000 rpm takes the efficiency from 94% to 90%
How much with an MG is an unknown but based on other motors and cars it can be very significant so assuming that the laws of physics have not changed, it is reasonable to assume that the MG motor is similar. Yes you are right that without the full picture we don't know and it can then make driving for the best efficiency hard.
The power gauge is a pretty good indicator of the efficiency as it reflects the power that the motor is consuming. I tend to use that when wanting the best efficiency.
As soon as the wheel size is changed, all bets are off. All the calcs go out the window. I have seen it happen on other EVs. I've measured it in my own converted EV that retained a gearbox where dropping a gear or two when low on the battery would reduce the load for a given road speed but limited the top speed. Electric motors like revs and low torque to be efficient.
It does seem more than coincidental that the FL bigger wheel cars are reporting less efficiency.
Equally it could be an aerodynamic affect as well. and a host of other factors, including rolling resistance, but I do think this efficiency aspect is a major contributor.
