Wandering Speedometer Needle at Highway Speeds

[pjmoreland]

After driving my truck a few times with the replacement speedometer installed, I noticed that it was off by 4mph at all speeds. Normally speedometers are off as a percentage due to incorrect drive/driven gears in the transmission, not a fixed offset, but this wasn't the case with mine. I selected the drive/driven gears using the TCI calculator, and I measured my tire circumference by placing marks on the ground as I pushed my truck forward one revolution of the wheels. I tested my odometer out on an eight-mile straight stretch on the interstate, and it was dead on compared to the mile markers. I noticed my speedometer wouldn't move away from zero until I was going about 4mph. Anyway, I took the speedometer apart and discovered an adjustment bar on the return spring. I did a little trigonometry and came up with needing to rotate the bar by 11 degrees CW (toward the 100mph end of the speedometer) to increase my speed reading by 4mph. This translated to moving the end of the adjuster bar by .164". I just got back from a test drive, and the speedometer is dead on now from 0 to 70mph. I compared it to the actual speed displayed while running Google Maps navigation on my phone.