if the 235 tyres are a 70% profile ie 235/70x16 then they will be the same rolling radius as the 205x16. the only difference is that the 235 tyre is 30mm wider. while this is not a problem as sutch, it is preferable to fit the same size all round. if you do fit the 235 tyres then they would need to be fitted to the rear of the vehicle. be aware that a 235/85x16 is a whole lot bigger and not suitable.