Smiths Gauge World Wide Sales & Service Center
most vehicles, the speedometer calibration is based on a certain tire and transmission
ratio. These factors were used by the speedometer manufacturer to establish the proper
combination of gears and worms inside the speedometer head to obtain accurate speed
readings. Some car manufacturers and individuals building customized cars are using
different tires and/or transmission ratios rather than OEM equipped. If this is the case,
the ratio has to be established. In general, if the tires or transmission, including
differential of your car has been or will be altered by you, or if any other changes have
been made that are not listed as original or factory approved equipment, the ratio has to
be adjusted. The following tests will tell you if the error you are experiencing is in the
speedometer gauge itself or is caused by cable delivery from the transmission.
After performing the test, call the APT Instruments tech line at
877-856-2103 with the results.
Conclusion: If odometer reads correctly per 10 miles driven, the error you are experiencing is in the speedometer head. If reading is incorrect, the error is from your transmission and the ratio must be corrected. Although rare, you could have both cable delivery and speedo head problems. In any event, your cable delivery must be correct in order for the speedometer to be accurate.
FORMULA FOR FIXING AMERICAN MADE CARS: Generally, American made vehicles with mechanical speedometers need 1000 cable revolutions per mile to make the odometer record correctly and the MPH to be accurate. Here's a simple formula for you to fix many American made car ratio problems. Conduct the 10 mile test, remove driven gear (the gear that spins your cable), count the teeth, and then multiply the teeth times the subtraction answer from the test. Example: 8.2 reading per ten miles actual multiplied by 38 teeth on driven gear (8.2 X 38) = 311.6, which means you need a 31 tooth gear to correct the error. In other words, 8.2 miles per 10 miles actual is 820 cable revolutions and you need 1000 revolutions to be correct. If you find that you cannot buy a driven gear with the right number of teeth, call APT at 952-881-7095 and they will build a ratio adapter to fix the problem.
THE HARD WAY TO TEST RATIO
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Last modified: May 14, 2013