just bought set of alloys (refurbed) for my daily car. fitted with 4x brand new tyres. cheaper brand tyres, so I was going to use them anyway till they wear out and replace them with decent stuff after . but just noticed that rears fitted with 255-40-R18 instead of 265-40-R18 as per manufacturer spec and current tyres on the car. (fronts are fine) so question is- can I run them as it is or to I need to change tyres to correct spec?
The tyres will be slightly smaller in diameter making the car sit slightly lower at the back, depends what you want to use the car for? Track car or daily driving miss Daisy. You will probably not notice so its up to you, I used to run a car with smaller tyres as previous owners wanted quicker acceleration, but that was all 4.
if there is a missmatch i am surprised it hasnt caused an ABS fault. and dont forget how it affects the speedo/road speed numbers
have you met an insurance estimator before? Most of them don’t know the difference between a Ducati and a Ducato
They are axle paired so mot won't be an issue. Technically the profile should have been 45 instead of 40 to keep car same height but as stated its minimal really around 5mm over the whole tyre so 2.5mm top & bottom if this makes sense. As for insurance dont worry, they only check tyre tread depth really. Just wear them down then buy the right size next, or if it bothers you then ring a few tyre places that do new & part worn & explain what the situation is & they can take your old tyres as a px against some new ones.
You may not keep them long anyway if this test is typical: https://www.tyretest.com/web/pages/summer-tyres/kpatos/fm601-4 Seem to work OK, but wear fast. The 8mm theoretical difference in diameter is probably less than the change from new to worn out on a standard tyre.
That would be quite a bit taller than the 265/40, but they probably don’t make 255/42 as a standard tyre!
yeah, as has been said sidewall size is a ratio of the width. I think you will have issues if for example you have tyre pressure sensors, as on BMW's they measure the wheel speed at the hub to calculate the tyre pressure (really clever way to do it but causes issues). My 2 series goes absolutely mad when a wheel is changed, I cracked a wheel and had to use a temp one and the car just lost it's sh*t... Also, at 10mm smaller the tyre is stretched on the bead, which is going to create a balloon effect on the tyre so it will wear them out quickly. Personally I wouldn't risk it, especially if it's not just a shite commuter.. what's the point.
In comparison its not all that bad, but me being anal i would change them. Swap them with the tyres off your used set if tread is still good. My BMW runs 275/30/19 rears & 245/40/19 fronts and wouldn't change them to anything other than spec tbh.
