Tyre sizing question.

mikewest

Just noticed last night that the spare wheel has a different size tyre fitted on what appears to be the same alloy as the reat of the car.
Spare is 205/60R15 all the rest are 225/60R15 same brand, rating etc and alloy looks identical. I didn't think to measure the rim depth as I was doing something else and tbh the difference didn't hit me until I woke up today . Not at home to check right now either.

Is it possible to fit the two different tyres on the same size rims. I know it is possible to go with some different widths e.g 175's instead of 165's in certain circumstances.

EPM

My dad for some reason when he gets a new car puts wider tyres on when the originals need changing. Probably wants better grip or something. Can't see ir making that much of a difference to be honest if only used until the original is repaired.

EDIT - Didn't do the maths there. Cheers mickdw

mickdw

The diameter of the alloy wheel is the R15 part of the tyre size so it would be the same size.
225/60/R15 means tyre width is 225mm. the 60 means the height of the tyre from rim to thread is 60% of the width so in this case is 135mm high.
For the 205/60/R15, width is 205, height is 123.
Now given that your rolling diameter is made up of the rim size (15 inches) and twice the tyre height(profile), the rolling diameter of the larger tyre is 651mm while the other one is 627 mm. This is too much of a difference and may well trigger a fualt if used on the car.

Its possible that your car originally had 205/60 tyres and someone replaced the 4 on the car with the slightly wider ones. No problem really. When you next get a new tyre or 2, stick one of the old ones on the spare.
You should check what the proper size is too as there will be an error in your speedo if the wrong rolling radius is used.
Generally, if you wanted to go with slightly wider than original fit, you would need to lower the profile. The suitable profile can be easily worked out as above to keep diameter right.

mikewest

Cheers folks. What had me puzzled was that both 205's and 225's are listed as recommended wheel sizes on the door pillar so I was wondering if maybe someone had switched spares in a garage sometime. I know I've had wider tyres fitted in the past on recommendation of the tyre fitter but I didn't know if there was gereral formula or way of working out what is acceptable. Spare needs replacing anyway.

mickdw

Is it 225/60 or 225/55 thats recommended. If its 60, well then just get the spare changed to that size

mikewest

205/60R15 and 225/60R15 if memory serves, plus others on different rims.