Rim Size and Effect on Speedometer

[DougC]

Given that the A4 can be optionally fitted with 17in to 19in rims (16in is standard), am I right in assuming that the larger the wheel rim size, the greater the speedo error would be? In other words, the larger the rim size, the more the speedo will tend to underestimate the true speed of the vehicle? Does anyone have any data on this and how much of an error it is likely to create?

:1zhelp:

[73kingstonroad]

Hi...

The larger the wheel is the smaller the side wall of the tyre is.
Over all the diameter of the tyre doesn't change that much. +-10mm is ok.
ie.
16" wheel will have a 185/70/16 tyre, 185 is the width, 70 is 70% of the the width and 16 been 16" wheel size.
were as a 19" wheel would have 235/35/19

Martin

[Crasher]

An increase in rim diameter must be compensated for by reducing the height of the tyre, the “aspect ratio” which is a percentage of the section width. The intention is to keep what is known as the “rolling radius” the same. As an example, a car fitted with a 15” wheel such as a Golf 4 with a 195/65 15 (RR 317-mm) can go to an 18” diameter wheel using a 225/40 tyre (RR 319-mm) and still keep a RR that is within spec.

[DougC]

Hi...

The larger the wheel is the smaller the side wall of the tyre is.
Over all the diameter of the tyre doesn't change that much. +-10mm is ok.
ie.
16" wheel will have a 185/70/16 tyre, 185 is the width, 70 is 70% of the the width and 16 been 16" wheel size.
were as a 19" wheel would have 235/35/19

Martin


Thanks for clarifying that, I have learned something. Appreciate it.

:beerchug:

[DougC]

An increase in rim diameter must be compensated for by reducing the height of the tyre, the “aspect ratio” which is a percentage of the section width. The intention is to keep what is known as the “rolling radius” the same. As an example, a car fitted with a 15” wheel such as a Golf 4 with a 195/65 15 (RR 317-mm) can go to an 18” diameter wheel using a 225/40 tyre (RR 319-mm) and still keep a RR that is within spec.

Ditto to my previous comment and thanks for the clear explanation.

Cheers

:D

[ini]

This tyre size 'rolling diameter' calculator is handy:

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

[DougC]

This tyre size 'rolling diameter' calculator is handy:

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


Thanks for that, a very useful tool for anyone considering changing rim size, tyres etc. Sorta confirms what I thought to some extent, although as a complete neophyte I didn't realise that the aspect ratio of the sidewall size had to be altered to maintain the diameter within an acceptable tolerance. As I said earlier, I have certainly learned something I didn't know before, so this has been very useful.

:beerchug:

[ScottyUK]

Since when you change rim size, you can't always get 100% match by changing the tyre size, I believe the Audi electronics allow them to compensate.

There's definately an adjustment for the SatNav (it uses this when the signal drops e.g. tunnels) so it would seem crazy for them not to add the fufdge factor into the settings to get it correct.

[DougC]

Since when you change rim size, you can't always get 100% match by changing the tyre size, I believe the Audi electronics allow them to compensate.

There's definately an adjustment for the SatNav (it uses this when the signal drops e.g. tunnels) so it would seem crazy for them not to add the fufdge factor into the settings to get it correct.


I wonder if anyone can confirm that? Seems logical.

:approve: