OK boys and girls, here's the information I promised about speedos for various
Triumphs. Turns out I only have a table for the TRs, but you should be able to
use the same method to figure it out for other cars.
First off, how to measure this stuff. The most important thing to know is how
your current tire diameter compares to the original.
(width Aspect ratio )
Overall diam(inches) = wheel diam + 2 x ----- x ------------
( 25.4 100 )
for example, wheel diam is 15, width is 185 and aspect is 70 for a 185/70-15
tire. (If the aspect ratio isn't mentioned, e.g., 165-15, use 78.)
Unfortunately, I don't know the formula for figuring out the overall diameter
of early tires that predate this numbering system (e.g., 5.50-15), but I'd
guess that the first number is the section width in inches, and the aspect
ratio is 78 or 80. Some early tires throw a letter in there as well, just to
confuse things more.
(There's a nice spreadsheet on ftp.team.net that does all this, btw.)
Axle ratios: or how to discover what your beast has without dropping the diff
to count the teeth. This method works well for open diffs; those of you dealing
with a limited slip unit will have to modify it a bit because of the way the
wheels work. (It also depends on fourth gear being direct; if yours isn't, use
the prop shaft instead of the crankshaft pulley.)
A. Remove the spark plugs
B. Wedge one rear wheel so it can't move
C. Handbrake off, engage fourth gear
D. Jack up the unchocked wheel to clear the ground
E. With chalk, mark the raised wheel and the crankshaft pulley, in order to
accurately count revolutions.
F. Have an assistant rotate the free wheel exactly 2 revolutions, while you
count crankshaft revolutions.
G. Repeat F to account for backlash.
The count of revolutions at G is the axle ratio. If you counted 3.7 revs to
your assitant's two, you have a 3.7 axle.
Why two revolutions? Because of the open diff - one rear wheel is static, the
other makes two, so the average is one. That's how an open diff works, it
splits up the drive so that
( L+R )
Engine revs = Gearbox ratio x Axle ratio x ---
2
For it to be reasonably accurate, your speedo should be the right one for the
particular rear axle ration that you have installed. There is a figure marked
on the dial, and this should be in agreement with the value show in the table
below - this represents speedo cable revolutions per mile. Owners of earlier
cars will find variations of +/- 2 (i.e., 1180-1184) depending on the tyres
originall fitted to the car. On the TR3, for example, the tyres were either
5.50/5.90 x 15
cross ply or 155-15 Michelin X; on the 4A they were either 6.95 x 15 or 165 x
15 radials of lower profile than either Xs or cross plies (no, I don't know the
exact profile, though I should go out and measure my original Michelin X spare
some day).
The speedo gear in the TRs is a 2.5:1 reducer. So if you take 800 revs/mile for
a 165x15 radial, a 3.7 axle and a 2.5 speedo gear ratio, you get 800 * 3.7/2.5
= 1184.
So, without further ado, the numbers:
TR Axle Original Triumph part Jaeger part Revs/km Revs/mi
ratio tires
3.7 5.50/5.90 108192 SN6319/00 - 1180
3.7 5.50/5.90 108193 /01 740 -
4.1 5.90 113632 /02 820
2 4.1 5.90 113631 /03 - 1312
3 4.1 155X 119046 /04 - 1344
4.1 155X 120206 /05 820 -
3.7 155X 119047 /06 - 1216
3.7 155X 120205 /07 740 -
4 3.7 165X/sp numerous unknown 735 1184
4A 3.7 6.95 numerous unknown 715 1152
3.45 165 numerous unknown 638 1100
3.7 165 not given 733 1180
5 4.1 165 812 1307
6 4.3 165 851 1371
4.55 165 901 1451
4.875 165 965 1554
Sorry for the holes at the end ... those of you with 185 tires stock on your
TR6 will have to do the calculations as homework!
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