Actually, the speed sensors do not calculate vehicle speed. The input speed sensor reads the rpm of the transmission's input shaft. The output speed sensor does look at the output shaft, (half shaft / drive axle), but then the computer knows which gear the transmission is in, the final drive gear ratio, and the tire circumference to calculate road speed. You did not say if your speedometer is off by two miles per hour, or twenty, and whether this just started or has always been this way. The final drive gear ratio and the tire size have to be programmed in so the speed calculations are correct. The first things to consider are whether the tire size was changed or a computer was replaced and its programming was not updated to your car. Next, depending on the symptoms and severity of the problem, there have been a lot of failures of the gauges' stepper motors. This could be a case of a failing speedometer gauge.
Your mechanic can use a scanner to perform a gauge actuator test. That runs all the gauges up to maximum, in steps, then back down to "0". That can show if the speedometer gauge responds correctly independent of what the sensors and computers are doing. If the speedometer works properly during this test, it can be eliminated as a suspect.
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Thursday, August 24th, 2017 AT 10:39 PM
