While a spacesaver spare wheel may have a significantly smaller diameter compared to its conventional wheels, it is also important to take into account the tyre itself, which on spacesavers is typically much taller and will help to reduce the overall size difference between the wheels. We would advise keeping the difference in total size between the spacesaver and normal wheels as small as possible - online stores such as the wheelshop.co.uk offer spacesavers up to 19-inches in size. If you must use a spacesaver that is significantly smaller than your normal wheels we would advise driving as short a distance as possible at low speeds and having your puncture repaired immediately. We would not expect this to cause an issue with your warranty, unless a suspension failure occurred that could be attributed to the use of the spacesaver. A bigger issue might be a disputed claim in the event of an accident if it is determined that the spacesaver was not suitable for use with the vehicle.