Hello - new to the list here - first post.
I have a rear axle ratio question on my TF - I read
the archives and in 1999 a similar question came up
but I'm hoping I might be able to dig a bit deeper
now.
I have a 54 TF 1250 - very original. I've always
thought all the ratios very numerically high (high rpm
at relatively modest road speed, and a first that is
almost useless on flat starts) but it was not until
this past weekend that I thought the ratio might be
even higher than usual.
I ran a bit of a road rally this weekend and we found
our odo fully 10% fast from the organizer's very
accurate distances. That suggests to me that the tires
are smaller and/or the rear axle is numerically higher
than the odo anticipates. The tires are 155R15 which
at a presumed 82% aspect ratio I put at 25 inches in
diameter. Stock was of course 5.50-15 which at a
presumed 92% aspect ratio I put at 25.12 inches. That
is a difference of only about 0.48% in circumference
which can't in itself account for the odometer error.
On the axle ratios, I understand that my TF should
have 4.875, and that the TD had 5.125. That in itself
is a 5% difference.
So then, should I presume that I have a TD rear axle
in this car? But then how to account for the
ADDITIONAL 4.5% error? Is it possible that I have an
odo calibrated for an optional 4.55 ratio and also
have a 5.125 rear end? That's about 11% (which is
actually a little too much?). When you got an optional
rear end did you get a different odo calibration?
Thoughts?
My understanding of how the odo works (as opposed to
the speedo) is that there is no room for error - it is
direct drive all through that system.
The only reasons I really care are:
1. I've always thought the ratios ABSURDLY fast and
many people have said "oh, that's just the old MGs!"
but I am not foreign to old foreign cars.
2. I'd like to go to a more leisurely ratio but don't
know what I'm at right now!
2. I am quite a serious rallyist (see
www.musketeerracing.com) and we're quite serious about
distances - I am likely to put a Halda or Aifab or
Langwell in this car for vintage TSD rallies and
calibrate it to a rolling distance, but I'd sure like
to figure out if there's something in the drive ratio
affecting the standard odo.
Thanks in advance for your help.
Andrew Comrie-Picard
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