A vehicle can have more than one recommended tyre size to suit different specifications. For example, on the Fiesta it may be that the 195/45 is suited to sportier models such as the ST-Line as the smaller sidewall provides sharper handling. Ultimately both of these tyre sizes will work well on your car. The 195/50 tyres will provide a slightly better ride quality than the 195/45s, so you can change this to suit your preference.

The MoT guidelines state that tyres on the same axle must be the same size, so as long as this criteria is met it should not be a fail, but we would recommend using the same aspect ratio on all four tyres.

The Smart ForTwo is an unusual case as it has wider tyres on the rear than at the front, this is to counteract that fact that it has a very short wheelbase and is rear-engined.

You can use the spare wheel for emergency use only without any safety concerns.