Speedo Reeding

[MICALEE]

Can anyone advise the difference in speedo reading when upgrading from 235/40/18 to 235/35/19?

I see a few threads stating %'s slower or faster - how is this worked out or is there a website which does it for you?

[s4marsh]

by rights it shouldnt make a difference to the speedo your increasing the wheel size but reducing tyre size so in effect the rolling radius or diameter i cant remember which it is now is the same, the easiest way is to record what speed you get per 1000rpm eg: 30mph or whatever you get and compare with the new wheels before buying if possible.

[MICALEE]

Well, the wheel diameter is increasing slightly - Standard=18" with 45 series tyre, upgrade is 19" with 35 series tyre. If Im right, to retain the exact size, a 19 would have to have a 30 series tyre.

[davs4]

Here you go..
http://www.miata.net/garage/tirecalc.html

[s4marsh]

good link dav,

[MICALEE]

Here you go..
http://www.miata.net/garage/tirecalc.html

Many thanks there Dav, just what I was after.

For ref for others who may wish to know, as its a popular upgrade, these were the findings:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/40-18 3.7in 12.7in 25.4in 79.8in 794 0.0%
235/35-19 3.2in 12.7in 25.5in 80.0in 792 0.3%