COUNTING THE MILES

from the BA7C Newsletter with many thanks

It is unlikely that the accuracy of the mileometer was ever intended to be better than “give or take 3%” but, if there is a serious mismatch between miles traveled and recorded miles, there is a chance that the fault lies within the gearbox or you may have the wrong instrument fitted. Apart from the early three speed gearboxes, all gearboxes have a worm and pinion on the output shaft intended to match the turns per mile (TPM) of the propshaft with the TPM required by the speedometer to make it notch up exactly one mile. If you want to check the system out, these notes may help.

 Speedo.  Quite a bit of useful information was pulled together by Ron Burchett in the 60s and this has appeared on the Motorpages web site www.austinseven.org.au. Also in the Technical Pages are dial face profiles which can be used to restore your speedometer once they have been downloaded to a printer. The TPM (revs/mile) for the more common speedos are:­

3360 revs/mile             Smiths CA & A - belt driven off tailshaft

2240 revs/mile             Smiths PA —as fitted with most 3 speed boxes— and also Smiths PN

1040 revs/mile             Smiths rotating drum, as fitted with most of the earlier 4 speed boxes.

Some speedometers have the TPM figure printed on the face of the dial. If in doubt, fit the speedo ca­ble to a hand drill and count the number of turns for a tenth of mile reading on the milometer. 

 Propshaft:  The TPM of the propshaft will obviously vary with the size of the tyres and the back axle ratio. Bill Williams’s book “Austin 7 Specials” includes several tables which enable calculations to be made of road speed versus engine speed for various combinations of wheel size, axle ratio etc. His table of Tyre Constants, reproduced from manufacturer’s data (and presumably for new tyres) shows the number of revs/mile made by the various sizes of tyre. The tyre constant is the circumference of the tyre divided into 63,360 (inches in a mile). The figure will vary slightly between manufacturers and between old and worn tyres. You may need to do your own measuring, including counting the number of turns made by the speedo cable when the car is pushed along for a few complete rotations of the wheels. Some of the quoted tyre constants below may prove useful:

  Tyres:

  Speedometer Gears:  Examples:- assuming that a three speed box will drive through a 9:44 ratio axle and a four speed box will drive through an 8:42 axle and both are on Avon Sidecar tyres:­-

 TPM of propshaft on 3 speed car will be 782 x 44÷9 =3823

Required TPM of a PA speedo =2240
Ratio of reduction gears in gearbox should be                 3823÷2240 = 1.71

Correct gears are 4 start worm with 7 tooth pinion   7÷4 = 1.75

  TPM of propshaft on 4 speed car will be 782 x 42÷8 =4105

Required TPM of a drum type speedo                           =1040

Ratio of reduction gears in gearbox should be    4105÷1040 =     3.95

Correct gears are 2 start worm with 8 tooth pinion   8÷2 = 4.00

  Other ratios are known, e.g. 5 start on 9 teeth. If you know where they should be fitted, please drop a note to the editor. Mismatching of pinions chews up the brass worm.  Sets of new gears may be avail­able from Vince Leek. His article on this topic gives more detail in Chris Gould’s “Building Ulsters”.  If the miles clock up OK but the speed appears wrong, the fault will probably lie with the instru­ment’s magnet.  Fixing this is tricky.  Anyone who can describe how to do it, please write in.

Article taken from the Bristol Austin Seven Club’s excellent newsletter with many thanks.