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Wheel Size and it's effect to your speedo...

Pva_Glue

By Pva_Glue,
in Engines & Forced Induction

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R31 POWER Mentor

R31 POWER

Posted
Posted

I think that to get the signal to the gearbox it has to go through the wheels first(of coarse),if they are bigger or smaller,it will change the amount of revolotions the wheel does.Think of it like diff ratio's.

Jay95R33 Grand Master

Jay95R33

Posted
Posted

That's it.

If the 18" tyres aren't matched right and have a bigger outside diameter, effectivly the bigger diameter tyre will travel further with one rotation.

So if your 16" tyre travels say.... 3 meters with one rotation, and your 18" tyre that isn't matched correctly travels 3.5 meters, then your speedo will be out by about 15%.

This is where you have to make sure the bigger tyre has the same outside diameter than the 16" one (the bigger the tyre the lower the profile).

There was this really good Java applet that I found on a web site one day that works it all out for you. I've got the link at work, so if I remember I'll post it up.

J

Mick Mentor

Mick

Posted
Posted

Yep exactly right, if u change size of rims yours speedo will be out, and if u change gearboxes it can go out as well changing to a dif type one i mean). U can take it to places that change the speedo gears to calibrate it, but a little expensive ...

But if u r like me and have an Apexi RSM, u needent worry at all, you can adjust wheel size on it to make the speedo spot on .. its even got where u can enter in how much tread is left so its dead dead on ... pretty cool huh ...

For the price of gettin speedo re-calibrated, u may as well get a RSM ...

blind_elk Grand Master

blind_elk

Posted
Posted
Originally posted by Jay95R33

That's it.

...So if your 16" tyre travels say.... 3 meters with one rotation, and your 18" tyre that isn't matched correctly travels 3.5 meters, then your speedo will be out by about 15%.

J

And it wil be out in favour of the police :eek: ie, the car will be travelling 15% faster than indicated by the speedo.

Silver-Arrowz Grand Master

Silver-Arrowz

Posted
Posted

It is important to get the ratio aspect of your wheels correct because it's what is of most concern to the police. OK, some cops say that having rims 2 inches over standard is illegal, but that's a load of s**t. Their concern is that by having incorrect profile will give you the wrong speed reading.

I'm not sure if this is the case in other states but I know that's what the RTA in NSW are most concern about.

Also be aware that having rims about 1 inch in width over standard, some insurance companies will not pay out on claims because

wider rims=wider tyres=aqua panning in the wet.

  • 2 years later...
cowie165 Veteran

cowie165

Posted
Posted

I know this is dragging out an old thread - I couldn't find anything more applicable and couldn't find an answer...

The RPS13 (I assume all S13's) has a trim pot on the back of the speedo that allows you to recalibrate the speedo yourself. After a number of hair-raising trips down the Barton Hwy in the ACT (note to self: just take your time by doing it when stationary rather than with a mate driving at speed) I had the problem fixed.

Does the R32 have a similar trim pot? Assuming that I am not going to match the rolling diameter of standard tyres/rims, I would like to save the cost of paying someone who has less of an idea than I do...

Any info appreciated, thanks

Mark

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