|The MGA With An Attitude
SPEEDOMETER TECH -- ST-101A
ODOMETER DRIVE Turns Per Mile Calculations
On 7/3/2016, Mark Hester wrote:
"I did not attempt to fit the new 3.7 (yes 3.7) gears myself. They would not fit into the 'Australian' MOWOG alloy housing without grinding and weakening the inside of the alloy casing. They did however fit into an old MGB banjo housing.
"How many turns per mile speedo do I need to go with this diff (3.7 ratio) and 15" wheels? I have 175/65/15 tires which I believe are approx 23.9" rolling diameter".
I looked up 175-65-15 ties on the Tire Rack web site, took the first 10 models on the list, looked at the specs for turns per mile, and the average is 867 tpm. Multiply this by the final drive ratio to find propshaft speed. Then multiply that by the speedo gear reduction ratio in the gearbox to find speedo cable drive speed.
867 x 3.700 = 3207.9 tpm (propshaft)
3207.9 x 5/12 = 1336.6 tpm (speedo)
Note that most of these tires are listed with 24-in diameter, but the actual rolling diameter works out to be 23.26-in. (average) When squashed under load the rolling radius is about 3/8-inch less than the unloaded radius.
For the Jaeger (or Smiths) speedometers, the primary odometer drive gear can be either 25 or 30 teeth (nothing else). Resulting odo reduction gear sets close to the desired ratio are:
25x53 = 1325
25x54 = 1350
30x44 = 1320
30x45 = 1350
The closest you can get would be 1325 tpm. This would make your odometer read 0.9% more miles than actually traveled. the 1350 tpm setup would read 1.0% less miles than actually traveled. Incidentally, 1325 tpm is the same setup I use with 165-80-15 tires and 3.909:1 final drive gearing. So going to 3.700:1 final drive just makes up for the lower profile tires.
After you wear 1/8" of rubber from the radius of the tire tread, the odometer will register about 1% greater distance, and the speedometer reading will also increase by 1% (along with engine speed). Most street tires have usable tread depth of 10/32 or 11/32 inch. Near end of tread life you maybe down 1/4-inch on tire radius, resulting in odometer and speedometer and engine speed increase of about 2% (for any given road speed).
With this in mind, you might prefer to use the 1350 tpm setup giving readings 1% low with new tires, spot on when half worn, and 1% high near end of tread life. But 1% low with new tires might get you in trouble with the speed traps. This depends on how well the speedometer speed readout is calibrated. For modern standards, government regulations may allow the speed reading to be up to 5% or 6% high, but never allowed to be lower than actual road speed. The speed reading may be calibrated independent of the odometer drive gearing, so you can choose the odometer output you like, and have the speed reading calibrated last.