Q. When my son bought his Seat Ibiza, the garage agreed to supply a spare wheel. The size code for the car’s tyres is 215/45 R16 but the spare is a 185/65 R15, which looks much bigger and barely fits into the well. Will it be safe to use?
DC, Eglwysbach, Wales
A. Sometimes spares don’t come in exactly the same dimensions as the other wheels — some manufacturers supply a smaller wheel with a larger tyre to save weight or cost, for example. It is important that the overall height of the spare wheel/tyre should be as close as possible to the one it replaces. Opinions vary as to how close in size the spare needs to be. Etyres (etyres.co.uk) suggests the height of the spare should be within 2.5% of the other wheels; Tyresafe (tyresafe.org) says motor manufacturers favour a limit of no more than 2% smaller or 1.5% larger.
The first numbers in your tyre codes indicate the width of the tyre in millimetres (215 for the current tyres and 185 for the spare). The second digit (45/65) is the height of the tyre wall as a percentage of the width. The final figure is the diameter of the wheel in inches: 16in, compared with 15in for the spare.
If you work it all out, you’ll find that the height of the spare is 621mm — 3.5% larger than the other four, so it falls outside all recommendations.
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory – read more from Dave here.
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