13 replies to this topic

[garypara]

hi does anyone know if you can put 235/45/17 tyres on the rear wheels cheers Gary

[Retset]

I reckon they would fit but rolling radius different so speedo will be out. Big question is 'why?'. A cm more rubber will not affect the handling much: much bigger tweaks can be made with a good geo.

[Darcini]

I have 235 R888s on mine, fitted to Thorney spec TD's. Retset is correct on the speedo being slightly out, approx 3mph slow at 40 on the speedo when compared to the scangauge reading. I got them brand new at a bargain price off Ebay, almost half the price of 225s Suspect this was a one off, though, but at 235 there are definitely more around as they are fitted to BMWs and the like. When you consider the actual size of the contact patch of a tyre any increase is good, particularly if you are running over 300bhp

[Mangham54]

I have 235 R888s on mine, fitted to Thorney spec TD's.
Retset is correct on the speedo being slightly out, approx 3mph slow at 40 on the speedo when compared to the scangauge reading.

That is a point I kept mean to explore... how does the Scanguage calculate your speed? I presumed it was just running off the same sensor that provides the feed for the Stack unit? Surely it can't calculate it off the rpm or crank sensor as it won't have the predefined wheel size to calculate it correctly.... Or am I missing something?

[SteveA]

Perhaps someone can explain to me why changing from OEM 225/45/17 to 235/45/17 effects the speedo

[techieboy]

Tyre has a bigger circumference, so for each turn of the wheel it covers more distance than a standard sized wheel and tyre. Therefore, the speedo under reads as it works on revolutions and not distance.

[MellowYellow]

Perhaps someone can explain to me why changing from OEM 225/45/17 to 235/45/17 effects the speedo

The "rolling radius" of the tyre will be larger on the 235 wide tyre, the 45 is the side wall section (as a pecentage of the width) eg: wall of 225 wide tyre is 101.25 (on diameter??) where as the wall of the 235 is 101.75, about 0.5mm difference which when calcualted gives about 28mm larger diameter, I think.
Here is an online calculator Tyres

[slindborg]

Adds 4.5mm to the wall size (so 9mm to diameter of the overall package)..... and that dosent take into account tyre pressures. so all in should make next to fcuk all difference. As for the scangauge vs speedo, they both use the same source for the signal so its just down to the dash being inaccurate (as they are legally permitted to be 10% upto 70mph and then whatever the hell they like there after). Even GPS is vauge as its not that good at keeping the velocity vector when changing altitudes EDITED TO BE ACCURATE COMAPRED TO MELLOWS BOLLOCKS SPOUTING SESSION

Edited by slindborg, 18 August 2010 - 12:24 PM.

[SteveA]

The profile of the tyre e.g. 45 is the bit that dictates rolling radius not the width e.g. 235 These tyres both have the same profile hence the same RR. ETA thanks for the info Mellow. I understood that the 45 was a set measurement not a percentage.

Edited by SteveA, 18 August 2010 - 12:22 PM.

[slindborg]

225/100 = 2.25, 2.25*45 = 101.25mm WALL 235/100 = 2.35, 2.35*45 = 105.75mm WALL total difference = 4.5mm on the RADIUS (or 9mm on the diamter) given tyre deflection, thats fcuk all

[swisscott]

235 45 17 ->

17 inch diameter -> 431.8mm
Plus 2 x 235 x 0.45 -> 211.5mm

Total diameter 643.3mm

compared to 225 45 17 ->
431.8 + 2 x 225 x 0.45 = 634.3

Ratio : 643.3mm/634.3mm = 1.014 -> 1.4% difference

[garypara]

235 45 17 ->

17 inch diameter -> 431.8mm
Plus 2 x 235 x 0.45 -> 211.5mm

Total diameter 643.3mm

compared to 225 45 17 ->
431.8 + 2 x 225 x 0.45 = 634.3

Ratio : 643.3mm/634.3mm = 1.014 -> 1.4% difference

[garypara]

235 45 17 ->

17 inch diameter -> 431.8mm
Plus 2 x 235 x 0.45 -> 211.5mm

Total diameter 643.3mm

compared to 225 45 17 ->
431.8 + 2 x 225 x 0.45 = 634.3

Ratio : 643.3mm/634.3mm = 1.014 -> 1.4% difference

Many thanks
Gary

[MellowYellow]

Perhaps someone can explain to me why changing from OEM 225/45/17 to 235/45/17 effects the speedo

The "rolling radius" of the tyre will be larger on the 235 wide tyre, the 45 is the side wall section (as a pecentage of the width) eg: wall of 225 wide tyre is 101.25 (on diameter??) where as the wall of the 235 is 101.75, about 0.5mm difference which when calcualted gives about 28mm larger diameter, I think.
Here is an online calculator Tyres

I was rushing as I was posting when I should have been working, meant to say 28mm increase on the rolling circumferance....sorry about the spouting of Bollox.