Tyre size for these rims that people are useing please ??

[clive]

18 x 8.0j           Front                 +30 Offset        

18 x 9.0j           Rear                 +35 Offset

[rx7veilside]

i think my mania vortex are that offset 235/35/18 on front and 265/40/18 rear but slight rub offside rear need to lift the suspension about 5mm on standard ride height will prob be fine

[Phil Bateman]

Forgive my intrusion but 265/40/18 are too big and will give a dia of 669 and speedo error. You need 631 dia or as near as possible to keep the speedo correct. 265/35/18 will give 642, that\'s 1/2" smaller on radius and probably will not rub.

Regards
Phil

[rx7veilside]

i prob got it wrong way round prob 235/40/18f and 265/35/18 rear

[TonyBlair]

Hi Phil,

I am right in thinking that whenever you change the diameter of your tyres then your speedo will need to be re-configured to reflect the change?

I remember some years my friend told me that they\'d seen 140mph on the speedo in his old and severly battered Calibre 2.0l (non-turbo). I found this hard to believe and attributed it to the fact that he had installed larger wheels. Yet... when I asked another friend about it he told me that changing the wheel size would not affect the speedo, was he right?

Cheers
Jon

[Phil Bateman]

If you increase the overall diameter of the wheel/tyres which connect to the speedo drive then the speedo will read low because the wheels rotate slower. So when your speedo says 70 you may be travelling at 75 +  depending on how much you have increased the overall diameter.
The bigger the wheel size the lower the profile to keep the same overall diameter, I am not aware of people having speedos changed.
Original Size
225/50/16 = 225x50%x2 + 16"(406) = 631 mm dia

New size
265/30/19 = 265x30%x2 + 19"(483) = 642 mm dia  = 1% error OK

Regards
Phil

[TonyBlair]

Thanks for the technical explanation Phil - much appreciated!



Jon.

[scooby_si]

yeah IMO always worth havign a play with a GPS device just to get an idea of how accurate yer speedo is for the purposes of ticket etc avoiding i reckon. Given speedos normally overread by as much as 10% (is legal extremity) then a slightly small circumference increase usually isn\'t too bad my 18\'s are just about on the money im speedo terms now....http://www.smileycentral.com/?partner=ZSzeb008_ZS\' target=\'_blank\'>http://smileys.smileycentral.com/cat/18/18_3_108.gif\' border=0>
Si

[Oliver]

Hi Clive, I\'m sure about the correct tire/offset for you but here\'s a nice tire calculator site I used when deciding on diameter

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

regards
Oliver