How is it cutting corners? Do you not understand there is more choice, less weight, better turn in due to less sidewall etc etc. I'm running Pilot super sports I don't see how that is cutting corners? What tyres do you run?
Because you are not buying the correct tyre size which was designed for the car?
So the speedometer, mileage, traction control, torque and gearing settings are all based on the distance that the tyre travels over one complete revolution. A tyre with a different outside diameter will travel a different distance over that one revolution with a different amount of torque.
For an extra £30 a corner why not put the correct size tyres on? It doesn't matter if they are bloody F1 tyres....if they are the wrong size its a waste of time....
I agree that it might not make 'that' much difference dropping the profile down by 5 but it is cutting corners
So the speedometer - Use a sat nav for true speed as the car is out +/- 3% anyway, mileage - not exactly a huge concern as the effect will be tiny, traction control - If you really have an issue drive without it, if you rely on it you shouldn't be driving the car, torque - is produced by the engine and not the tyres, so the effect is miniscule and gearing settings - unless you're going from a 15" alloy to a 18" alloy the difference will again, tiny! are all based on the distance that the tyre travels over one complete revolution. - Over a mile the 45 profile will rotate 756.3 times, on the 40 profile the tyre will rotate 784.6 times. Not a huge difference! A tyre with a different outside diameter will travel a different distance over that one revolution with a different amount of torque.
Speedo differences for you:
20 - 19.3
30 - 28.9
40 - 38.5
50 - 48.1
60 - 57.8
70 - 67.4
80 - 77
90 - 86.6