tire circumference

On a road-trip last weekend, I did some comparison between the
car's trip-meter and the highway mileposts. On various highways
with different maintenance administrative structures, just in case
one state's set of mileposts are less accurate than another's. From
what I hear, mileposts are supposed to be dead-accurate to begin
with. Anyways, I found that the car's odo would gradually drift
behind where the mileposts were, i.e. if the milestick went by at
.2 miles it would be going by at .1 and then .0 maybe 20 or 30
minutes later. This is on the stock Integrities, right up near
their rated inflation [42/40 or 42/42].
.
This means that the tires were turning just a tiny tad too slowly.
.
While groping around tirerack.com a while back, I found that the
Integrities are rated at 855 revs/mile, while the Michelin Hydroedge
is rated at 856 revs/mile. That's all of six feet per mile -- not
quite enough to make up for the milepost disparity, but it seems
like the Hydroedges *could* contribute to making the odo a little
more accurate.
.
_H*

 

This thread got buried long ago, but to talk about tires and rates
of rotation and related things I figured I'd just bump it. A new
project here is looking into doing some differential wheel-speed
analysis with an eye toward building a simple low-tire warning
system of some sort -- despite the fact that the NHTSA has rejected
the indirect TPMS concept and wants carmakers to use the direct-
measuring systems for all new vehicles. Hmm, when *all* cars are
spewing forth four wheels' worth of RF data, are cars next to each
other going to start picking up each others' pressure readings?
Early results from the differential analysis are promising, at least
for a simple visual indicator of speed deltas -- but determining
any sort of yes/no alarm condition would take a *lot* of coding.
.
Unrelated, I took a ride in a friend's Murano today. The tires on
that are also rated for 44 psi but he only had 32 in all of them,
as *recommended* by Nissan's door placard. That's idiotic. Since
I had my air pump while out doing my own pressure-test runs I offered
to pump up all his tires to the correct pressure, which would have
been quite a lot of work with a hand pump... he's not worried about
it, though, since he's not getting particularly stellar mileage
regardless.
.
_H*