Here is a good link that allows you to compare tire sizes and the how
different tire sizes will affect your speedometer.
http://powerdog.com/tiresize.cgi
R. Ashford Little II
www.geocities.com/ralittle2
-----Original Message-----
From: owner-triumphs@autox.team.net
[mailto:owner-triumphs@autox.team.net] On Behalf Of Randall Young
Sent: Thursday, May 15, 2003 12:31 PM
Cc: triumphs@autox.team.net
Subject: RE: 6" A/R Wheels + Tire Size
> Most good tire dealers have a specifications book that gives the
> circumverance of tires. What you need to do to keep your speedo
correct
> is buy a new tire size with the same circumverance as the stock tire
> size. You can move between 60 and 70 series as you wish as long as
that
> dimension remains the same or very close.
They are also available on-line in most cases, many of them are at
http://www.tirerack.com/tires/SearchTires.jsp
For speedo calibration, you want to look at the "turns per mile" figure,
as
different tire constructions sometimes have different amounts of "squat"
which changes the effective circumference even for the same advertised
size
tire.
To find the original turns per mile, find the "engine rpm at 10mph"
figure
for 4th gear direct in your owner's manual. Divide this by the final
drive
ratio (3.7 for most big Triumphs including the TR250) to get tire rpm at
10
mph, then multiply by 6 to gets turns per mile (which is the same as rpm
@
60 mph).
So for example my Stag owners manual (which happens to be laying open at
the
moment) gives 514 engine rpm @ 10mph. This means the original tires
were
approximately 833 tpm. (Tire size is always an approximation, it
changes
with rim width, tire wear, inflation and even temperature.)
Randall
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