Selecting your Series II/IIA/III Speedometer
They aren't all the same you know. If you have the
wrong one fitted it will never be accurate. Luckily, it is quite simple to figure out if
your instrument is right for your truck and what tyre sizes will agree with it.
Series II/IIA/III Land Rover Speedometers were
made by Smiths-Jaeger and came in four basic varieties, if you ignore the presence or
absence of resettable trip odometers.
(1) Series II/IIA Metric
(2) Series III Metric
(3) Series II/IIA Imperial
(4) Series III Imperial
Which can be further subdivided into
(1) Ones meant for 88s with stock crossply tyres of anything from 6.00x16 to 7.50x16
depending on the year, market, or special customer order.
(2) Ones meant for 109s with stock 7.50x16 crossply tyres or 9.00x16 crossply tyres on the
One Ton model.
I believe that there never were any optional
speedo drive gear ratios down at the transfer case end of the speedo cable. The drive
gears invariably had a ratio of 2.2: 1 from propshaft to speedo. Therefore the matching of
speedo to tyres is done by changing to a different speedo or a different tyre size.
I do not know what the situation was for the
1960's US market models which came from the factory with tyres on 15 inch rims. I have
heard rumours that they had optional speedo drive gears available for the transfer case
output housing but I have no solid information. For the benefit of North American
visitors, I would appreciate any authoritative information for these pages on the tyre
sizes fitted to these US models and the optional speedo drive ratios if any.
The only critical difference between a II/IIA
speedo and a III speedo is that the earlier speedo had a metal screw-on fitting for the
speedo end of the cable outer whereas the later speedo had a plastic push-on fitting. The
speedos can be physically interchanged if you use the right cable for the speedo. The
fitting at the transfer case end is identical. The Series III speedo did have idiot light
bezels on the bottom of the face for oil pressure, high beam and cold start so a II/IIA
speedo being pressed into emergency service in a III (maybe just to get the truck
road-legal) would need to have these lights relocated elsewhere as well as a II/IIA type
speedo cable. Going the other way, a Series II/IIA owner could use or ignore the Series
III light bezels as he saw fit.
Deciphering your speedo (at last!)
- Each Smiths-Jaeger speedometer is internally geared to turn over
one complete unit on its odometer for a given number of turns of the input shaft. This is
known as the "turns per mile" or "turns per kilometer" for the
particular instrument. The
speedo needle is calibrated against the odometer to indicate the correct distance over
time. Any changes to the turns per mile/kilometer has to be done by an instrument repairer
with a stock of spare parts. If you are sure that your odometer is reading accurately, any
inaccuracy of the speedo needle can be checked and adjusted by a repairer on the test
bench.
- The turns per mile/kilometer is marked on the speedo face as part
of the identification code. Sometimes it is printed above the odometer window, sometimes
close to the bottom edge of the face where it is hard to see.
A Metric Series III speedo I have in front of me
is marked SNT-6209/OLAS 940 down at the bottom of the face.
The first eleven characters are meaningless to me but the 940
is the turns per kilometre.
Like every stock Series Land Rover I've ever seen, the truck this
speedo came out of has a 4.7 : 1 reduction diff and a 2.2 : 1 speedo drive
gear reduction from propshaft to speedo. 4.7 divided by 2.2 = 2.136 which means that we
have a 1 : 2.136 step up from rear wheels to speedo. For every revolution described by the
wheels we get 2.136 revolutions of the speedo. Remember that 2.136 as it is a constant (K) that you will use in every
speedo calculation.
turns per kilometre divided by K = turns of rear wheel per kilometer
SO
940 divided by 2.136 = 440
Our truck will do 440 turns of the rear wheels for every
kilometre clocked up on the odometer.
1000 metres divided by 440 wheel turns = 2.27 metres of travel per
wheel turn
OR
a wheel circumference of 2.27 metres.
Circumference of wheel divided by 3.14 (close enough to Pi) = diameter of wheel.
SO
2270mm divided by 3.14 = 723mm or 28.46 inches.
28.46 inches is not a very tall tyre so you could bet that
this speedo came out of an 88 and you would be right.
Lets try the same trick on another Metric speedo I
have which is marked 880 turns per kilometer.
turns per kilometre divided by K = turns of rear wheel per kilometer
SO
880 divided by 2.136 = 412
So our truck will do 412 turns of the rear wheels for every
kilometre clocked up on the odometer.
1000 metres divided by 412 wheel turns = 2.427 metres of travel per wheel turn
OR
a wheel circumference of 2.427 metres.
Circumference of wheel divided by 3.14 (close enough to Pi) = diameter of wheel.
SO
2427mm divided by 3.14 = 773mm or 30.4 inches.
This speedo came out of a 109.
Here are the calculations for an Imperial speedo I have which is
marked 1408 turns per mile.
turns per mile divided by K = turns of rear wheel per mile
SO
1408 divided by 2.136 = 659
Our truck will do 659 turns of the rear wheels for every mile clocked up on the odometer.
5280 feet divided by 659 wheel turns = 8.012 feet of travel per wheel turn
OR
a wheel circumference of 8.012 feet or 96.14 inches
Circumference of wheel divided by 3.14 (close enough to Pi) = diameter of wheel.
SO
96.14 inches divided by 3.14 = 30.6 inches.
Very similar to the metric speedo in the last example.
Another 109 speedo.
Here are the calculations for an Imperial speedo
marked 1540 turns per mile.
turns per mile divided by K = turns of rear wheel per mile
SO
1540 divided by 2.136 = 720
Our truck will do 720 turns of the rear wheels for every mile clocked up on the odometer.
5280 feet divided by 720 wheel turns = 7.33 feet of travel per wheel turn
OR
a wheel circumference of 7.33 feet or 87.96 inches
Circumference of wheel divided by 3.14 (close enough to Pi) = diameter of wheel.
SO
87.96 inches divided by 3.14 = 28 inches.
This will be an 88 speedo.
Just to make things more complicated, the novice speedo researcher should not take the calculated wheel
diameters too seriously when deciding on which tyre will best suit his speedo.
The wheel diameters we worked out above are really just a rule of
thumb method for deciding whether you have an 88 speedo or a 109 speedo. Any speedo whose
wheel diameter comes to more than 30 will probably be out of a 109. Anything less than 29
will be from an 88.
Fortunately, you established a wheel revs per mile/kilometer figure for each of the
above speedos in the very first part of each calculation and you can match this with the
wheel revs per mile/kilometer quoted for each tyre in the tyre manufacturers data book.
If you do do some arithmetic on a data book you will notice that the revs per
mile/kilometer you can calculate from the nominal tyre diameter is not quite the same as
the revs per mile/kilometer quoted by the manufacturer. Thats because a rolling tyre is
made of rubber and is not a perfect circle. It's flat on the bottom when loaded so the
effective radius of the loaded tyre is less than the radius of a brand new tyre lying on
the ground. The amount of flattening is dependent on the section width of the tyre, the
recommended inflation pressure, the aspect ratio of the tyre and the sidewall stiffness.
The engineers take all of these things into account when working out the revs per
mile/kilometer.
Here's two manufacturers who have the easiest-to-read tyre data tables
on the 'Net.
BFGoodrich® Clever site and you
can save .pdf files containing high quality images of the data tables.
Cooper Tires , owner of the Avon tyres name and
also retailing Cooper branded tyres.Simple, fast loading site with full data tables.
Heres a table showing some tyres matched to the speedos we worked with
above.
TYRES |
SPEEDOMETERS |
Size |
Rec.
Rim
(inches) |
Overall
Diameter
(inches) |
Overall
Diameter
(mm) |
Turns per
Mile |
Turns per
Km. |
88 Imp. Speedo
suits tyre
720 turns
per Mile |
88 Metric Speedo
suits tyre
440 turns per Km |
109 Imp.
Speedo
suits tyre
659 turns
per Mile |
109
Met.
Speedo
suits tyre
412 turns
per Km. |
| 205R16 |
6.0 |
28.5 |
724 |
719 |
447 |
MATCH |
MATCH |
- |
- |
| 6.00x16 |
5.5 |
28.6 |
727 |
726 |
451 |
MATCH |
MATCH |
- |
- |
| 225/70R16 |
6.0 |
28.3 |
719 |
735 |
456 |
MATCH |
MATCH |
- |
- |
| 235/70R16 |
7.0 |
28.9 |
734 |
720 |
447 |
MATCH |
MATCH |
- |
- |
| 7.50R16 |
6.0 |
32.1 |
815 |
654 |
406 |
- |
- |
MATCH |
MATCH |
| 235/85R16 |
6.0 |
32.2 |
819 |
646 |
401 |
- |
- |
MATCH |
MATCH |
| 265/75R16 |
7.0 |
31.8 |
807 |
654 |
406 |
- |
- |
MATCH |
MATCH |
NOTE: The tyre
data above is greatly condensed from typical tyre manufacturers tables.
It is only intended to illustrate this exercise and should not be used as a buying
or fitting guide.
Refer to the far more detailed manufacturers tables at your local dealer or on the
Internet. |
These are excellent matches. The worst of them is a little more than 2%
from perfect. In practice, you can learn to tolerate a speedometer which reads up to 5%
out either way, but be sure to check your local transport laws first.
Now that you know how it is done, you can work out whether a speedometer
is a match for the tyres you want.
If you have no idea what tyres you want, you know how to select a size which won't upset
your speedo.
If your speedometer matches your tyres, yet is wildly inaccurate you know it is time to
seek out a repairer.
And finally, do your homework first with the tyre data books but always
listen to the advice of the professional tyre fitter.
This page last updated on Wednesday, 17 February 1999 10:28
|
