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Hi Devs!!!My suggestion is about the wheel rims.Are we going to be able to have a different rim size for the front and rear car wheels?For example,a supercar has a 19 inches rim at front and a 20 inches rim at the rear.

That is not planned, no. How large a portion of production cars have different sizes rims?

is it even allowed when passing MOT (or your national equivalent) ?

It is here. As long as they fit over the brakes using DOT certified wheels, they are legal.

Most modern supercars have a larger wheel diameter in the rear than in the front (like 18" front, 19" rear). You could say it's a supercar norm these days. I don't think this is really needed in the game as it would be basically a styling-only thing.
As for wheel width, Porsche began fitting wider rims to the rear in the early '70s. But currently I believe this happens automatically when you adjust tyre width, so not really needed as a separate option.

Legal here in NZ too. AFAIK you can use whatever the hell you like as long as rolling diameter is no more than 5% larger than stock, so if you want to run 18" on the fronts and 14" on the back you're more than welcome to do it if you get the right tires.

In real life a larger tire diameter gives a larger contact patch between tire and road, all other things being equal. High performance cars compound this by generally having rear-biased weight distribution, so they tend to be equipped with a larger rear wheel diameter than on the front.

The game does take tire profile and wheel diameter into account when calculating stats, so this could be worth looking into.

Ferrari, McLaren, Koenigsegg, Pagani, Bugatti all have larger wheel diameter at the back than the front. Along with many other supercars. But I don't think this feature is strictly necessery for this game....