Jump to content
SAU Community

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

Pva_Glue

By Pva_Glue,
in Engines & Forced Induction

 Share

Recommended Posts

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

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing


  • Similar Content

  • Latest Posts

    • soviet_merlin
      RH9 R32 GTR Build Thread

      By soviet_merlin · Posted

      Out of curiosity, how do you cover up the inspection hole? Surely it doesn't sit open like that, right? With grime and dirt coming in and oil going out?
    • Waders360
      M35 Brake caliper replacement

      By Waders360 · Posted

      Hi from Canada, Both of my calipers are starting to fail. My car is a 2005 M35 Stagea ARX and I can't seem to find any parts or any cross reference caliper that would fit my car. Is there a company or a similar caliper that would fit and I could replace the OEM one. Or if anyone knows where I can find a new OEM. Any help is very appreciated. Let me know!
    • 25zenki
      S1 rb25/s14 starting problem

      By 25zenki · Posted

      Maf is a question mark but TPS  I have set at 0.47, the fact that I’m getting proper voltage range out of it with the key on leads me to believe it’s functioning properly. I’m getting proper voltages on basically everything. I still need to look up voltage ranges for z32 maf and test that 
    • 25zenki
      S1 rb25/s14 starting problem

      By 25zenki · Posted

      Honestly no not at all, but it’s a very basic tune, it’s stock injectors q45 tps and z32 maf. Other than that it’s a completely stock base map. The file is from nistune and from my understanding if it was tune related I would be able to unplug maf and tps and it would still start even if it was a really shitty start. I have tried starting it with coolant temp, maf, and tps unplugged none of those 3 togethor or individually changes anything 
    • Matt Z
      SAU NSW - Bathurst Run 2025

      By Matt Z · Posted

      🏆 Skyline Supremacy Meets Mount Panorama Magic 🏆 Join SAU NSW as we take the long way to Bathurst, the home of GTR greatness. Saturday 30th August 2025 7:00AM Meeting Hanna Park North Richmond 7:30AM Departure Cruise Via Bells of Line Road 9:00AM Arrival to Stop 1: Golden Poplars Meadow Flat 9:45AM Arrival to Final Destination: Mount Panorama Motor Racing Circuit Meet Location: Hanna Park North Richmond Stop 1: Golden Poplars Meadow Flat Final Destination: Mount Panorama Motor Racing Circuit   *Disclaimer* There will be a lead and follow car so no one should get lost. If you would like to attend or bring others along please put your name down and a +1 as numbers will be needed prior! This is NOT a race and we will all be adhering to all road rules. If this is what you want please come to one of our many track days. This is an official SAU:NSW event and will be run under a CAMS permit. One of the things that really sets our club apart is our commitment to being true enthusiasts. When on normal roads we strive to maintain good relations with the authorities as well as the public in general. When attending one of Skylines Australia NSW events please try to: • Be aware of surrounding environment and act accordingly. • Drive courteously on the state’s roads as a true enthusiast should. • Understand how important it is to maintain the good name of SAU NSW and thus, treat others accordingly. • Any misbehavior will not be tolerated and you will be asked to leave.
×
×
  • Create New...