Land Rover Speedometer Ratios

[Yorker]

Which speedometer have I got?



The identifying mark on the face: SNT 6209/22 1504

The calculations

Series Land Rover have a 4.7 : 1 reduction differential and a 2.2 : 1 speedometer drive gear reduction from propshaft to speedometer. 4.7 divided by 2.2 = 2.136 which means a 1 : 2.136 step up from rear wheels to speedometer. For every revolution described by the wheels we get 2.136 revolutions in the speedometer. This is a constant relationship (K) that can be use in every speedometer calculation.
Here are the calculations for my Imperial speedometer marked 1504 turns per mile.
turns per mile divided by K gives the number of turns of the wheels per mile

1504 divided by 2.136 = 704

My Lightweight will do 704 turns of the wheels for every mile clocked up on the speedometer.

A mile is, 5280 feet, divide this by 704 wheel turns gives 7.5 feet of travel per wheel turn

that is a wheel circumference of 7.5 feet or 90 inches.
Dividing this by 3.14 (close enough to Pi) gives the diameter of wheel.

90 inches divided by 3.14 = 28.6 inches.

Hence, it is calibrated for 6.50 x 16 Tyres.

LWB speedowill have the figures 1408
SWB ones are 1536 or close.
Some are 1376 if intended for bigger tyres.

These figures are speedo revs per mile.

[RyuKyuRvr]

So how do we best adjust the guage to more accuretly read for my tire size. I have a 7.50x16 guage and a 6.50x16 guage but my new tires are as tall as 9.00x16's. Do I need to find a 101 guage?

[EricWS]

So how do we best adjust the guage to more accuretly read for my tire size. I have a 7.50x16 guage and a 6.50x16 guage but my new tires are as tall as 9.00x16's. Do I need to find a 101 guage?

You need to have the gauge re-calibrated. Call Nisonger. 101 parts can be very rare over here and expensive over there...

[greenmeanie]

If a 101 speedo is what you need they are quite freely available in the UK. Parts #s are:
Parts book: PRC1057 reads km/h with mph inside. Should be 85gbp
MPH version: RTC5034 Approx 121GBP depending on supplier. It also does a S3 with 600/650X16 tyres
KM/HR alternative version: PRC2605 also fits the S3 mentioned above at approx 99GBP.

I don't know how that compares to Nisonger.

The last two are not official 101 parts but work perfectly well in the application. I think the difference is that the original has the three warning lights, oil pressure, high beam and cold start in the face of the gauge like a SIIA. The SIII items do not have that IIRC.

There are some 101 parts that are body specific and a few mechanical items but the a large amount of the detail parts came straight out of the LR parts bin. A large amount of the mechanical parts also came from external suppliers who used them on other vehicles. They are not as easy to keep as a standard Rover but a little time spent researching, cross referencing and building your knowledge of suppliers in the UK gets you most things.