I adjusted the aspect ratio of the 165 tires to match the published TR4 TPM. I can not explain the rather significant deviation of the calculated speedometer calibration from the speedometers that were used in the cars. If I use their "Rolling Radius", the 165 radials and bias tires would have aspect ratios of 77 and 74 respectively. Under that circumstance the calculated calibration is even further away from the supplied speedometers. If I work backward from the stock speedometers, I get an aspect ratio of 90 and 85 respectively!
Please note that these numbers above are for descriptive use only. You need to test each set of tires at your chosen inflation pressure and load to determine the actual turns-per-mile and speedometer calibration.
EDITORS NOTE: A favorite method of a "do-it-yourself" turns/mile determination is to find a level spot of empty parking lot, make chalk marks 52.8 feet apart. Get one, or two friends to help you push the car , with the tires and inflation you intend to use, between the two chalk lines a few times. Use some tape on the body to judge you have traveled the distance. You should disconnect the speedometer drive cable and use some masking tape on the protruding inner core to wrap and make a "flag" at the end. Count the number of turns (including fractional turn at the end) a few times until you get a pretty good average, or run out of beer for your friends. Three times ought to do it, if you are careful. 52.8 feet is 1/100 of a mile. If you multiply the turns average you count (about 9.8 on the Tiger) by 100 you will get the turns per mile of the drive line. If this matches the number on your speedometer (unlikely) and your speedometer is accurate (unlikely) you are in business. If not, you know what the rebuilder must have to calibrate your speedometer. If your rebuilder is local (unlikely, except in the Los Angeles San Fernando Valley and MOMO or Nisonger Instrument (NY) . The stock Tiger number (except Mk II) is 980. This number is higher than the example table taken from Triumph data, but the methodology is the same, if you use the correct numbers for the differential, etc.
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