The rear tires on my 135i are bald and ready to be replaced. I want to just by rear tires and convert to non-runflats.

The tire folks of course want to sell me 4 tires so they say they can't be mixed. The car would handle terrible.

The rears are 245/35/18 and the front are 225/40/18.

Has anyone mixed with sized like this and if so what were the results in terms of car stability/drivability?