Tuesday, October 14, 2008 - 04:01 pm, by: David Vaughan(Davidv)
According to the site specs.amayama.com the stock wheel and tyre for a 1995 JZZ31, whether or not with G package, was 215-15/60. That makes the diameter 639 mm.
However, the 225-16/55 was also a factory option and its diameter is 654 mm, up 2.7%.
Tuesday, October 14, 2008 - 04:35 pm, by: Michael Fabian(Mf_sc300)
Thank you SO much David!!!
I just installed an RSM and its got this feature to show the correct speed if u have changed tyres. According to my speedo i am doing 100 but i always seem to be speeding past everyone. So this should tell me what speed i am really doing.
Thanks again!!!
Peter Nitschke Junk Filterer South Australia UZZ30 UZZ31
Tuesday, October 14, 2008 - 07:49 pm, by: David Vaughan(Davidv)
Borrow a GPS Michael (or iPhone with GPS app) and go for a steady run on a freeway under cruise control. That will tell you in practical terms what correction you need, or check the setting you made based on the tyre measurements.
I have 225/55/16 tyres on our JZZ-31 and I can tell you the speedo error is under 1km/hr right up to 130km/hr (checked against GPS). This means that our original 15" tyres were under-reading on odometer and optimistic on speedo (not checked against GPS but the Miata calculator).
From the calculator, your change would result in a 3.6% under-read on speedo (100 reading, actual 96.4) which may or may not be a good thing- depends on your current speedo error. This difference is similar to the difference between your current 16" tyres and mine (225/60R16 vs 225/55R16).
So to get a close to accurate speedo you should be looking for a tyre which gives you close to my diameter, which is as close to accurate as is possible. I'd suggest, your choice of 17" tyre does this, it seems, with only 0.2% difference between your tyre choice and my actual.
Hope this helps.
Don Bagnall Moderator New Zealand Mercedes Benz SLK230 Kompressor
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