On a Mk1 Nissan LEAF downsizing from the Tekna 215/50 17 to the Acenta 205/55 16 makes a huge difference, around 5% with the same model and brand of tyres. Admittedly based on a sample of 1, and with Goodyear EfficientGrip. It may be different for others.
It's impossible to say whether it's the...