Tyre Size - Advice?

[starseven]

Hi, first post so apologies if it's in the wrong place.

I'd like to know if there are any benefits (or safety issues) in using tyre sizes that aren't standard for the car?

Basicaly, my old Clio came with 4 x 185/55/R15 tyres, but the standard size is 175/65/R14. I drove it for 3 years on these tyres and didn't seem to have any real issues.

I've now got a Rover 25 and the same situation: 4 x 185/55/R15s and the standard is 175/65/R14. Before I start buying new tyres, can anyone give me any advice? Would like to find out for sure if it's generally safe to do.


Thanks.

Edited by starseven on Thursday 5th December 14:02

[littleredrooster]

You're going to get loads of opposing views on whether or not it is safe. I won't comment on that, but would observe that a 185/55 x 15 is most definitely not a straight replacement for a 175/55 x 14. It has a significantly larger rolling radius and will affect the gearing, handling and the speedometer accuracy.

Are you sure you have the sizes correct?

[kambites]

I suspect in both cases the smaller wheels were "standard" and the larger ones that you had were a manufacturer fit option. There's a pretty significant difference in rolling radius, though.

ETA: From reading around, 185/55/15 seems to be a manufacturer fitted size on the Rover 25.

Edited by kambites on Thursday 5th December 13:57

[kambites]

It looks like you have the figure for the 14s wrong, for the Rover at least. The 14 inch option was 175/65/14. Anyway, your current tyres are correct; replace them with the same size.

[Krikkit]

185 55 15 and 175 65 14 are perfectly interchangeable, the difference in rolling radius is about 0.25%, so speedo etc won't be affected.

[starseven]

Thanks kambites, you're correct - I did have the Rover size wrong (and edited the original post). Also, I've noticed the sizes I found on blackcirlces.com were for the 1.4, I've got a 1.6 and the standard size for that is 185/55/R15. The Clio size is right though.

littleredrooster - thanks, I had no idea it could affect speedometer accuracy. Looks like standard sizes are there for a reason.

Thanks guys, much appreciated.

[kambites]

Krikkit said:

185 55 15 and 175 65 14 are perfectly interchangeable, the difference in rolling radius is about 0.25%, so speedo etc won't be affected.

Indeed. The OP originally said 175/55/14, which is rather further off.

[starseven]

Krikkit said:

185 55 15 and 175 65 14 are perfectly interchangeable, the difference in rolling radius is about 0.25%, so speedo etc won't be affected.

cheers, I'll bear that in mind. Probably stick with standard sizes now but always good to know.

[kambites]

Your speedometer just counts how often your wheels go around, so obviously if the tyres are bigger overall, they'll go round less times for a given speed and hence your speedometer will think you're going slower than you actually are.

The way the tyre sizes work is thus: 185/55/15 means 185mm wide tread; 185*(55/100)=102mm high sidewalls and 15 inch wheels. So the overall diameter of your tyres is 15 inches plus twice 102mm. That number needs to be kept consistent, or as close to it as possible.

Edited by kambites on Thursday 5th December 14:13

[Andy616]

kambites said:

Krikkit said:

185 55 15 and 175 65 14 are perfectly interchangeable, the difference in rolling radius is about 0.25%, so speedo etc won't be affected.

Indeed. The OP originally said 175/55/14, which is rather further off.

At the risk of a Parrot, goodluck changing them!

[B'stard Child]

Andy616 said:

At the risk of a Parrot, goodluck changing them!

clever