Getting an accurate tire rolling diameter can be tricky. For instance, if
one were to measure the outside edge of the driver's side front tire of my
'87 Civic Si, you'd get a distance a good inch or two larger than the same
measurement taken on the inside edge.
As you may recall, I recently purchased a Honda CRV for my wife, so the onerous
chore of driving that peppy little black Honda fell to me. And I was lulled
into the trap of treating it as a transportation appliance, though it is 15
years old, with well over 100,000 miles on the clock. Lately it was wandering
on the highway, wanting to jump lanes on bumps, you had to constantly work the
steering, et cetera.
Long story short - after I noticed the shiny steel wires sticking out of the
inside edges of the front tires where the tread used to be, I investigated.
Using sophisticated, high-tech tools [1 x 2 firring strip with a couple of
paint stirring sticks tacked to it as pointers] I reduced the 25 mm of toe out
to something much closer to the 0 toe called for in the factory specs. What
a difference! It steers like I remember now, and with the drive wheels headed
in the same general direction as the rest of the car, it accelerates quite a
bit better too.
I think I have an old set of 185/60/13 Yoko 8Rs somewhere that I was saving for
a Spitfire emergency. I'd say the steel belts poking out of the tires on one's
daily driver counts as an emergency ;-)
As for the accuracy of the speed calculations, standing around the bbq after a
day of running at Wendover, I'm more likely to spout "Yep, I was doing about 90
going into the east sweeper" instead of "Yep, I was doing 87.932 going into
the east sweeper"
mjb.
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