Tyres, changing tyrewall size?

[MGZRod]

G'day.

Needing new gyres and ones I have on are 205/45/17.
Just wondering whether I'd be able to put on ones with a wall size of 40 or is it generally not done?

Car is an MG ZR with 17" straights.

[hyperblue]

Yes you can do it, it'll make the ride slightly harsher though.

[E30M3SE]

Your probably better off going 215/40 17.

[MGZRod]

Will it make that much of a difference to speedo? Wouldn't have thought it'd be too daft, not like it's putting on 13" wheels?

Saves me £40 a tyre and need them asap.

[Tom74]

E30M3SE said:

Your probably better off going 215/40 17.

This. Side profile is not an absolute figure, its a ratio with the width. Going wider will keep a rolling radius closer to the OEM one and push the speedo out less.

[Mr Sparkle]

Tom74 said:

E30M3SE said:

Your probably better off going 215/40 17.

This. Side profile is not an absolute figure, its a ratio with the width. Going wider will keep a rolling radius closer to the OEM one and push the speedo out less.

Agreed, I have done this on cars before, just calculate the tyres rolling circumference and then find a width/profile combination that matches it. If you're not sure after you can always check the speedo with a gps device.

[MGZRod]

I'm not too clued up on wheel things so apolagies if this sounds stupid. But will 215 ones fit on the standard wheels.

[E30M3SE]

Shouldn't be an issue, it's only 10mm wider. What width are your wheels?

ETA, a quick google suggests they are either 7 or 7.5", so if that is the case the 215 will be OK.

Edited by E30M3SE on Tuesday 13th September 17:31

Edited by E30M3SE on Tuesday 13th September 17:31

[ludicrous speed]

This might actually bring your speedo closer to a true reading, it wont neccesarily push it in the wrong direction.

[MGZRod]

Intriguing. 215/40/17 are about £72 for a T1-R. Not too bad, get saving then exhaust. Meh.

[TallPaul]

I'd expect there to be at least as big a variation between different manufacturers "same" sized tyres as there would be in changing from a 45 to a 40 profile tyre.

[Nick3point2]

TallPaul said:

I'd expect there to be at least as big a variation between different manufacturers "same" sized tyres as there would be in changing from a 45 to a 40 profile tyre.

Bingo!

The difference will be less than the difference between new and worn tyres so I wouldn't worry.