Some vehicles have a staggered wheel and tyre setup where typically the rear tyres are larger than those at the front. This is often seen on performance cars, especially those that are rear-wheel-drive, to ensure sufficient traction on the driven wheels, but also some larger SUVs. If the tyre dimensions from the factory are 255/45 rear and 235/50 front then we would strongly recommend that you maintain this arrangement. With tyres that are narrower than specified on the rear you are reducing the size of the contact patch and therefore the amount of grip, which could upset the vehicle's handling balance. Additionally if the car is four-wheel-drive, fitting incorrectly sized tyres can cause wear issues with the drivetrain.