> 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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