At 10:02 AM +1000 13/5/98,
john_worthy_at_wca-0606@mail.workcover.nsw.gov.au wrote:
> Previously I asked the lkist for assistance in calculating the overall
> gearing of a TR6. The question stemmed from the fact that the TR6 I
> recently purchased had 185/65 tyres on it. Got a number of responses
> to my question and all agreed that 185/65s were totally inappropriate.
>
> I now want to replace these and was set on getting a set of 205/70s
> until I read on the VTR maintenance page an article on TR6 tyres which
> says:
>
> "I would stay away from the 205/70-15 or shorter profiles (like 60
> series) unless you don't care about how accurate your speedometer is.
> If you're into autox you might want the 205/70 because they are
> roughly 2% smaller diameter."
>
> This person promoted the 215/70. Now I'm totally confused.
>
> If the original tyres were 185/80 then the tyre width was 148mm.
> 205/70s are 143.5 while 215/70s are 150.5. It would seem 215s are
> closer to original. However. they border on overtyring the rim and
> look big. What advice can listers give me?
>
> John
> 72 TR6
John
ISTR that radial tyres with unspecified profiles are 78 rather than 80. If
that is correct, the original tyre height was 144.3 rather than 148.
The rule of thumb that I was told many years ago is to go down one profile
size for each increase in width. That is: from 185/78 (or 80) to 195/75 to
205/70 etc. You can also check the radius with a tyre supplier if you want
to be more accurate.
I have 205/70 on my car, and they run well without fouling anything. I am
told that you can fit 215/65, but I have never seen it.
As for speedo accuracy, how accurate do think they were to begin with and
how accurate are they now after 25 years? Do you use the tyre pressure
recommmended by Triumph? The usable tyre tread is about 5 mm deep, so the
radius will change by that much during the life of the tyre.
Trevor Jordan
74 TR6 CF29281U
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