Just bought a Skoda superb with 17" alloys fitted with 225-45-17 tyres. The spare wheel is a 16" steel with a 205-65-16 tyre fitted. Can anyone tell me if the circumference is the same being different profiles? I thought the 17" wheels were standard until I saw the 16" spare.

Lesser specced cars did come with 16's with that profile tyre. I myself have the same setup. 17" rims with 16" spare. Not sure if rolling radius is exactly the same, seems close enough to me though.

Lesser specced cars did come with 16's with that profile tyre. I myself have the same setup. 17" rims with 16" spare. Not sure if rolling radius is exactly the same, seems close enough to me though.

225/45r17 diameter is: 63.43 cm

the other one is: 67.29 cm

So the difference in rolling circumference is 12.1 cm, or an increase in speed of 6%.

It would have made more sense to have 205/60R16 as this would have given a diameter of 65.2 cm, and about half the speed difference of the 65 profile.

I would not run with a 6% speed difference on the same axle as this loads up the differential and will no doubt lead to steer when fitted to the front axle, but as a temporary get you home feature, it is ok

Having changed my wheels a fair few times I have found the following links fairly useful............ :yes:

Visual Tyre Size Difference:

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

Wheel Fitment, Tyre Stretch, Rolling Radius and Speedometer Error Calculator:

http://www.willtheyfit.com/index.php?