Andrew:
If you can buy/borrow a portable GPS, you'll be able to measure your
speed to an accuracy of 1/10th mph. You can also use it as an odometer, and
of course it will show you where you are any where in the world. (It's like
magic!)
The rear ends of the T series are really short, even for the period in
my opinion. I have a TD with the stock 5.125 rear end. It takes 4500 rpm to
go 60 mph. Does anyone know of any other car manufactured in the '50s with
gearing that short? But, I have cruised many thousands of miles at that rpm,
and the engine keeps coming back for more. I never cruise beyond 60 mph,
because that exceeds some rule of thumb about not sustaining a piston
velocity above 2500 ft. per min. Has anyone else heard of this piston
velocity rule?
I'm really curious about how fast other T owners cruise. Please reply.
Bob Donahue (Still stuck in the '50s)
Email - bobmgtd@insightbb.com
52 MGTD - NEMGTR #11470
71 MGB - NAMGBR #7-3336
----- Original Message -----
From: "Andrew Comrie-Picard" <musketeerracing@yahoo.com>
To: <mg-t@autox.team.net>
Sent: Tuesday, August 26, 2003 6:44 AM
Subject: TF rear axle ratios
> 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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