Will increasing tyre size mess up speedo readings?

jgoater, Apr 8, 10:28pm
I want to remove my 14inch alloys from my Subaru and put on 17inch ones.
Will this mean my actual speed will be higher than the speedometer shows?

pandai, Apr 8, 10:32pm
Depends on the size of the tyres. For instance, 195/65/14 is roughly the same circumference as 225/40/17. But the wider tyres may not fit on the car, and will alter the handling.

tintop, Apr 8, 10:34pm
Yup - if the new setup has a greater circumference than the old.

Measure the rolling radius of the two sized wheel/tyre setups.

Divide the larger by the smaller to get the multiplier number . It will be 1.xxx

Multiply your speedo reading by that number with the new wheels to get your actual road speed.

mrfxit, Apr 9, 1:22am
OR
Be lazy & just use a decent tyre calculator designed for this job
http://www.tacomaworld.com/forum/tirecalc.php

rob_man, Apr 9, 3:09am
An increase of 5-10% will probably bring your speedo up to what it should be reading anyway.
edit. make that down to what it should be.

jokerboss, Apr 9, 3:43am
here's two links in one, Subaru car club & a car calculator tool handy for a few tings http://clubsub.org.nz/forum/showthread.php?590-Car-Calculator-V2-1-download-HERE!-Convert-KW-gt-HP-rolling-diameters-and-MORE!

lookoutas, Mar 22, 12:46am
Mine is on 215-45-17.
You don't want to increase the rolling circumference, otherwise you will knock the crap out of performance. And the speedo reading.

Your 14" tyres should stand about 24.12" high.
My 215's should stand at 24.73", but in fact measure 23.75.
If you keep at about that height, the effect on the speedo will be minimal.