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wheel circumference
mat.price - 4/10/08 at 06:22 PM

As the title
I have a 14" alloy wheel on my Avon with 185/60 r14 tyres on it!
I would like to put 15" alloy wheels on it but i would like to keep the some wheel circumference as i have a mechanical Speedo and dont want to put it out!
How do I get round this, tyres wise?

Paul TigerB6 - 4/10/08 at 06:25 PM

Use a lower profile tyre to give the same rolling circumferance. Probably want a 55 profile tyre - there are a few websites that give you the info you want. Toyo springs to mind

Edit to say a quick check gives a Toyo CF1 185/60/14 as 578mm. A 195/50/15 is also the same diameter at 578mm and just happens to be a commonly available size with plenty of options for sticky rubber such as R888's

[Edited on 4/10/08 by Paul TigerB6]

smart51 - 4/10/08 at 06:42 PM

You want to look up the rolling circumference rather than the diameter. The rolling circumference is calculated from the number of turns per mile rather than by putting a tape measure round it.

Paul TigerB6 - 4/10/08 at 07:04 PM

here's a size calculator

Agree on the above yeah. Rolling circumference will be slightly lower due to the deflection in the tyres than the measured circumference. Still think you are perfectly safe to use the above calculator to match it up using the diameter though - the differences will be very very small once the rolling radius is worked out and well within the error allowed with the speedo i'm sure.

Still recon 195/50/15's are the way to go using the calculator.

[Edited on 4/10/08 by Paul TigerB6]

tomblyth - 4/10/08 at 07:06 PM

look here there's a size calc!

[img]http://www.alloywheels.com/tyrecalc.asp[/img]

BenB - 4/10/08 at 09:52 PM

There's also a rolling circumference calculator (amongst other things) at the tyre bible. Just type that into google and (for once) you'll find what you're looking for!!!!

02GF74 - 5/10/08 at 09:31 AM

quote:
Originally posted by Paul TigerB6
here's a size calculator

Agree on the above yeah. Rolling circumference will be slightly lower due to the deflection in the tyres than the measured circumference. Still think you are perfectly safe to use the above calculator to match it up using the diameter though - the differences will be very very small once the rolling radius is worked out and well within the error allowed with the speedo i'm sure.


yep - using circumference will mean speedo will over read, whcih is what you want.

mat.price - 5/10/08 at 09:45 AM

ok people thanks for the info thats just what im looking for