Quick tyre size question for RS4 B5 alloys.

[Dan Gliballs]

Have I got the correct size on my 18's with 225/40/18R-Z's?

That's what I have on there already, and I'm just about to order four T1-R's in that size.

I notice my speedo over-reads by about 4-5mph at the moment, so just wondered if the last owner had got it right

Cheers

 

[Randomjim]

Yea 225/40/18 is correct. If anything it should under read though (10% slower than the speedo reads)

 

[Dan Gliballs]

*Yea 225/40/18 is correct.

*If anything it should under read though (10% slower than the speedo reads)

*Ahh, excellent!! All ordered for Monday now.

*Errrm, that's the same thing isn't it? LOL When my speedo says 80mph for example, the Road Angel reads around 76mph.

 

[Randomjim]

Oh right yea thats it you knew what i meant

 

[AndyMac]

But that size is actually a slightly bigger rolling radius than stock, so it actually makes the speedo more accurate. On stock tyres your speedo would read over 80 with RA on 76mph.

 

[Dan Gliballs]

But that size is actually a slightly bigger rolling radius than stock, so it actually makes the speedo more accurate. On stock tyres your speedo would read over 80 with RA on 76mph.

Has anybody measured this?

It's just that when I had some 15" alloys on a Vectra, and then switched to 17's with low profile tyres, the circumference was actually 1/4" LESS than with the 15's on high-profile tyres

 

[Randomjim]

i had 205/55/16 as standard an i went to 225/40/18 and the 225's are about a centimeter bigger in diameter ish

 

[AndyMac]

you don't need to measure it just use one of the tyre size calculators, enter the stock size and the new size and it calculates it for you. It all depends on the stock size and then ultimately what new size you go for. If the new size is wrong for the car then of course you'll get a bigger difference on rolling radius from stock. They recommend no more than 5% either way.
On an A4 stock is 205/50/16 = diameter of 24.1"
The closest 18" tyre is 205/40/18, but most go for 225 = diameter of 25.1"
So about a half inch bigger all round.
http://www.rochfordtyres.co.uk/tyrecalc.asp

 

[Drizz]

Ive just bought some 18s with no tyres, so thanks, i was wondering the same!