cschollum
02-12-2006, 12:42 AM
# Each 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
# 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