944S2/968 max speed ?

[thom]

It read 265kph down the autobahn but the road tests read 240kph . Is the speedo slightly erratic? If so how can it be fixed without being replaced?
Thanks

>>> Edited by thom on Sunday 17th February 22:46

[Leithen]

Sounds about normal Thom - many speedo's are up to ten percent optimistic, especially at the larger end of the scale...

All part of Le Grand Conspiracy.....

[thom]

Ah...
And I suppose that fitting bigger wheels affects even more the phenomenon, doesn't it ?

Thanks Leithen
(and your dad is a great chap indeed...)
thom,
the knight who says 'ni'

>> Edited by thom on Sunday 17th February 23:37

[Leithen]

Not an expert on the wheel side Thom, as far as I know just fitting larger rims doesn't usually change the rolling radius.

The consequences of increasing or decreasing the radius on your speedo is much too complicated for me to work out..... that would require logic after all - no place for that in the world of silly walks and cheese shops....

[kevinday]

Thom, If you fit wheels with a larger rolling radius then your speedo will read a lower figure for the same road speed than before, this will be directly proportional to the change in radius (or circumference). If you fit larger rims you usually decrease the profile to keep the radius about the same. Also remember if you fit wider rims of the same radius (e.g. 8" instead of 7") with wider tyres you also need to reduce the profile. The profile is a percentage of the width of the tyre. Simplest way to check what you are doing is to set up a spreadsheet, including wheel size and tyre sizes to calculate the rolling circumference. Then you can do what if? changes and see the result before you buy any new kit.

[thom]

great kevinday, thank you for your science!

[Pete Cros]

This may help you
www.miata.net/garage/tirecalc.html

[marky]

athough I don't question the science, changing your wheels to make your speedo read correct seems a bit bizzare !!!!!

Usually its the other way round (if people are that bothered), after putting bigger wheels on get the speedo corrected.

Also you have to assume that the error is linear, ie a 10percent error at the top end is mimcked in the middle,

my advice if it bothers you that much get a GPS, drive at 240 kph, take the speedo to bits and bend the needle till it reads correctly , personally I only ever lookat my speedo as I approach cameras.