Yes Bob, the tire size is directly proportional to the amount of error in
your speedometer. The speedometer is set to use a specific tire size to be
close to accurate, via the gear drive in the tranny, and to the ratio for
the final drive. Any changes to that ratio effects the accuracy of the
reading. Changing the gearing in the diff or the changing of tire and wheel
sizes directly affects your readings. You need to find out the diameter of
the tires that were originally installed on the car and then you can have a
tire shop find the closest match in a lower aspect tire.
Since most of our cars came with 83% aspect tires you will be able to follow
a pretty good rule that for example, the original tire is 175/rX15
you can go to a 185/sr/75 X 15 and achieve a close match on diameter. Or a
195/sr/70 X15 or even a 205-60X15. These tires will all be with in .25 inch
of the same diameter. Then your biggest concern will be clearance, wheel
well clearance at full compression on the rear and rubbing
suspension parts on the front. But you don't have to run the same size
tires fore and aft if you will not be rotating them. So you could run
195-70s on the front and 205-60s on the rear. Or the appropriate sizes that
suit your needs.
JMHO
Larry Zink
Z Group Racing and Performance
Houston, Texas
----- Original Message -----
From: Bob Danielson <rdaniels@snet.net>
To: Triumph <triumphs@autox.team.net>
Sent: Tuesday, August 03, 1999 6:47 AM
Subject: Speedometer and Tire Size
>
> I'm currently running 185/70s on my TR6. On the trip up to VTR and back my
> wife was in the family car following me, registering a very different
speed
> from what I was showing.... I'd be cruising at 70 only to find out it was
> more like 60 - 63 on her speedometer. We had the same problem when
following
> VTR directions for various events that would have you zero your tripometer
> and then go 4.8 miles and turn right........... even with the tripometer
I'd
> register a higher reading ....... maybe 5.0 instead of 4.8 miles. So I'm
> guessing that I'm consistently high by 10 - 15% or so. How will tire size
> affect this error? Will a 60 series (lower profile) make it worse? Going
to
> a 205 shouldn't matter as they're only wider.... right? I know that
> re-calibrating the speedo is another answer but first I need new tires.
> Thanks in advance.
> Bob Danielson
> 1975 TR6 - current status at
> http://pages.cthome.net/BobD
>
>
|