Tiger's
There has been a lot on the list about speedometers and speedometer
gears. Let me add my own experience in this subject to try to
illuminate the issues we are faced with.
Firstly, there are many factors determining the most correct selection
of ALL the factors that affect the Tiger speedometer AND odometer readings.
1) The Speedometer indication can be adjusted with re-magnetization of
an age reduced drag magnet and reaction spring that makes the needle
read a specific number at a specific cable rotational rate. This can be
done over a wide range of incorrect values for the rest of the driving
chain (below). Usually it is accurate at the calibration speed
indication chosen, and falls off on either side. This is normal even
with correct driving set-up, but gets worse the more you divert from the
correct set.
2) The "correct set" is what makes the odometer and trip indicator read
accurately. These are gear driven, not magnet/drag driven, and are only
adjustable by correct gearing.
3) Correct gearing usually starts deviating when non stock tires are
used. Since they are usually larger in diameter (unless you are a low
rider), the speedometer and odometer read slower/lower than actuality.
Correct restoration of both indicators require some gear train changes.
4) What "gear train"? Let's start with the thing you see the most of,
the speedometer. The British, bless their unique system, alter the
speedometer gears for every model that is different, even within the
same company, line, and car. The Mk I, IA (if Norman will allow the use
:-) ) have a very small set of numbers on the lower edge of the face.
They should read "980". This speedometer, REQUIRES 980 turns of the
input cable for every mile traveled to make the internal gearing and
spring/magnet balance read properly. Change tire size, and you don't
make it. Quick test. Find flat paved land and strong, weak willed
friends. Measure off 52.8 feet of distance and mark the beginning and
end. This is 1/100 of a mile. Set a chalk mark on the tire sidewall to
match the ground mark. Disconnect the speedo drive cable at the speedo
head, and let it lower to below the dash. Place a "flag" of masking tape
on the end of the cable so you can see it. Push the car until the
bottom of the tire touches the opposite ground mark, while counting the
number of turns the cable makes, including fractional turn at the end.
The tire mark at the start will NOT line up with the 52.8 foot mark, so
don't worry.
You should have counted 9.8 cable turns. If you did not, then somewhere
(or everywhere) in the drive train there is a wrong size or count. The
% error can be calculated, providing your speedo is functioning
correctly in the first place.
5) Gear Train pieces. Tire diameter (rolling effective), rear end gear
ratio, helical internal gearbox speedo drive, speedo cable end mating gear.
Just to show you why I know such trivia, it is due solely to having put
a Mk II gearbox in my Mk I. The ratios were terrific. Who would have
thought that Chrysler-Rootes, in all their wisdom, would have changed
the speedo calibration to require 1750 turns per mile!!!!! The internal
gearbox helical drive gear had changed, and I could have used the Mk I
correct version if I had known. Terrible way to learn all this junk.
6) Everybody understands the rear end ratio change influence. The
gearbox is another beast, as just described. There is an internal
helical gear (plastic and large) inside the gearbox. This gear drives
the speedo cable end gear that MOST talk about. Both of these gears are
available with very few options. The internal one looks like it has more
teeth than a shark, but they are helically twisted and hard to count. I
understand there are 5 to 7 teeth! The cable end has 15, 17, 18 teeth.
Each color coded. The correct combination will give you the 980, or 1750
revs/mile required. If they do not, or you don't want to remove the
gearbox, strip it, and replace 1 plastic gear, you have a problem.
Magnetization will fix the speedo needle, but the odometer will be some
20% high. Makes the gas mileage look good, but rally calculations
become difficult. It is almost a certainty that the speedo gears can
not be changed to the newer ones, as these old gears are difficult to find.
American speedo industry has been standardized, it seems almost forever,
on 1,000 turns/mile. All other components are sized by the manufacturer
to make this be the appropriate value. Naturally, our cars are
"something completely different".
OK, long treatise on small subject. The discussion about changes to
gear boxes, tires, rear end ratios, cables, speedo errors, etc.
galvanized me to try to describe the "chain" of events that one understand.
Another solution to this mis-match has long been used in the trucking
industry. They make a transfer box with internal gears that are
changeable for compensation. I used one on the RED TOI custom car
pictured in my article some 23 years ago. It is not a large, but should
not hang off of the speedometer. You will need to place it between two
shorter cables that have had speedo fittings installed. It should be
placed where the cover and fittings can be removed to change the
internal gear set if your set-up is modified. They are not all that
expensive, and available through most truck parts stores. I went to a
manufacturer here in L.A. Since we have stock American fittings, it is
no sweat.
Good Fortune, and the only person you fool with a bad speedo is
yourself, not the cop with the radar gun ;-)
Kathy and Erich Coiner wrote:
>----- Original Message -----
>From: "Frank Marrone" <itswonderful@attbi.com>
>To: "Tiger List (Tiger List)" <tigers@autox.team.net>
>Sent: Friday, September 13, 2002 11:11 PM
>Subject: RE: Speedometer gear - got one for 3.07s
>
>
>
>
>>Frank
>>B9471116 (in need of a speedo cable gear for 3:31 gears)
>>
>>
>
>Frank,
>
>My tiger has either 3.23 or 3.31 gears. When I bought the car it still had
>the 15 tooth factory gear. Speedo was reading way fast. I had an 18 tooth
>gear in the tool box, (left over from my attempts to get the speedo correct
>in a mustang after installing a toploader) This made the speedo read too
>slow. I checked it on an odometer check along an interstate. I purchased
>the 17 tooth gear that Sunbeam Specialities sells as a MkII gear. Now my
>speedo is accurate within 2 percent.
>I am not sure of the gears I have. The previous owner said they were
>3.23's. I have not seen that ratio for sale just 3.31. The PO should know
>because he owns a jeep/offroad supply store and he used gears for a jeep in
>it.
>
>Erich
>
>
>
--
Steve Laifman
Editor
http://www.TigersUnited.com
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