Tyre sizes to adjust speedo!!

[Azman]

Hi guys.

What tyre size is needed to get the speedo a bit more accurate??

I am running 225/40/18's at the moment and the speedo is about 7mph off at 70mph (according to the trip and sat nav)

Increasing the rolling road will make the speedo more accurate so how much off a difference would 225/50/18's make or 235/45/18's.

Anyone got any idea.

Cheers

Aaron

[Azman]

Also anyone know anywhere or the best place to get my wheels checked for shape. I seem to have wheel either off balance or out of shape. They don't look out of shape but I am having one hull of a problem sorting this out.

cheers

[Azman]

Anyone??

[dan_b]

I think all speedos are generally anything up to 10% off - I like your thinking of increasing the rolling diameter of the tyres a touch- I'm not sure of the maths though! Also, tyres from different manufacturers are not necessarily the same actual size even if they have the same size indications on them, and of course as your tyres wear they become smaller...

Any reputable tyre shop should be able tell you if you've got any pothole damage to the rim - it's very common in the UK unfortunately. This can be balanced out to some extent but it depends on how deformed the wheels have become.

[Crawford_S2]

personally i quite like the safety margin you get with a over optimistic speedo, its like setting your alarm clock fast:-) if you want a true reading get gps i say.

[mcandmar]

I'm confused is it reading too slow or too fast? The change from 205/60/15 to 225/40/18 should be 1.6%, 70mph on the dial is 68.8mph road speed.

Have you fitted an S2 speedo? I noticed a change when i did which i think made it read slower again, never got to the bottom of that. Either that or the rev counter changed which i dont think, 3000rpms in each gear gave a different speed by 2-3mph.

[neebover]

My speedo is 5mph out, e.g sitting at 80, the trip computer will show 75mph as does my Road Angel..

[Fat Audi 80]

I would't worry and just live with it. It is possible to have the speedo calibrated, a la police cars, but not worth the effort.

Cheers,

Steve.

[Stick Insect]

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

try this link regarding sizing.

HTH

David

[Greg_S]

see this thread http://www.s2forum.com/forum/showthread.php?t=14910