I'm ashamed to say that its taken me more than 4 months to notice my spare wheel/tyre.

I have 18" wheels with tyre size 225/40/R18, and yet the tyre on my spare (which I speced when ordering the car, and I believe had to be a `spacesaver`) is 205/55/R16.

Is that right? Isn't it going to cause some sort of `imbalance` should I ever have to fit it in the event of a puncture?

Many thanks.

Mike.

205/55R16 is the standard size of spare as far as I'm aware. It is the size of mine and I got mine new (it's for an Octavia Scout). Can't imagine the factory including something that is dangerous, but mine does have yellow speed warning stickers all over it so as long as you adhere to the warnings guess all will be well.

Yes Mike, that’s the size they supply for the Superb and Octavia.

Yes Mike, that’s the size they supply for the Superb and Octavia.

Thanks, Mate.

225/40/18 has the same rolling radius as a 205/55/16.

225/40/18 has the same rolling radius as a 205/55/16.

Thats really confused me. I can, and do, accept that its okay to use...but not that 18" is the same radius as 16"

Thats really confused me. I can, and do, accept that its okay to use...but not that 18" is the same radius as 16"

If you remove your spare from the boot and stand it next to your wheels on your car it will stand the same hight and width. the tyres/rim may be different but it's the sidewall of the tyres that are different so it compensates for the different wheel sizes

Does that make sense

If you remove your spare from the boot and stand it next to your wheels on your car it will stand the same hight and width. the tyres/rim may be different but it's the sidewall of the tyres that are different so it compensates for the different wheel sizes

Does that make sense

I think you're saying that the sidewall of the `spare` is 1" more (taller/deeper) than the tyres on the car, making the circumference 18" in total. I get that. I'll check it out tomorrow. Thank you so much for the explanation.

I think you're saying that the sidewall of the `spare` is 1" more (taller/deeper) than the tyres on the car, making the circumference 18" in total. I get that. I'll check it out tomorrow. Thank you so much for the explanation.

This 'calculator' may also be of help.

http://www.miata.net/garage/tirecalc.html

TP

I do find it strange that wheels and tyres mixed the units between metric and imperial measurements - no wonder people get confused.