- >Just took the C out on the freeway. Seems the speedometer is 10 MPH fast(reads 80 really only going 70). Somewhere I have seen a chart with the different speedometer pinions that go with the different rear ends(assuming that is the problem). Can't find it any more. Anyone out there have this information? I'm running 235X75X14 tires. I don't think this would be the problem. Each tire size and each tire design will revolve a certain number of revolutions in one mile. A speedometer, on the other end of the rear axle gearing and the transmission speedo drive and driven gears, is looking for 1000 (lets use that number to agree with another post that may well know what Chrysler std was in those days; my memory is that that standard number should be 1020 revolutions every mile). Speedo gears make this change. The speedo cable in a car should therefore revolve an easy to measure 37 7/8 revolutions in a straight ahead measured 200 feet or half that in 100 feet. If the number comes up short in number of revolutions, the pinion would need fewer teeth and so forth. A speedometer shop can build ratio boxes to make corrections with the above collected data in hand. We just bought one for a truck and it cost us about $60. I have done this and it works not only in theory. The tires have to be correct and inflated properly and the odometer will change calibration as the tires wear from the calibration point. This revolution per mile calibration will help check and calibrate the odometer. The speed registry will have to be checked and calibrated off the car on a test stand. My experience has been that my speedometers are way off after extended down time. This I would surmise is because of a number of things in the speedometer head assembly but the magnet/speed cup relationship is where I have felt the problem was centered. I had a real talented fellow go through my C300 speedometer and it did work just fine right after calibration only to change some after a long down time. HTH Warren Anderson Sedona,AZ