Hello Andrew,
to find out which ratio your car has you must jack up one rear wheel, turn it
ten times counting the revs on the prop shaft and divide that by two.
(original TF: 97.5 / 2 = 4.875)
I recomment to fit a bicycle computer. That little unit will give you the exact
speed, distance, average and so on and it is much cheaper than a Halda.
If I know right the odos on the MGs were not calibrated themselves but
different
ratios at the speedometer drive were fitted.
If you prefer to drive the original gearbox I recomment to fit the 4.3 ratio
from the MGA. If you like to have 5 gears you should fit the Ford Sierra
gearbox
with the 4.875 ratio. That is the optimum.
Dieter Wagner
Andrew Comrie-Picard schrieb:
> 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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