Can anyone explain this. Just got my 74 Challenger on the road, and the Speedometer is way off. Using my GPS which is pretty accurate according to my other cars for the MPH, showing 60 on the challenger Im actually only doing about 48 MPH.
Understand, that the car has a 2.76 open axle. I have verified this be looking at the tag on the transmission, as well as performing the rotation test (10 tire rotations gives 13.75 rotations of axle x 2 = 27.5 so it is a 2.76. Also car came from factory with 318/904 and MYMOpar says that the 318 cars and auto came with 2.76.
Car has on it 235x60x14 inch tires which by the calculators equals 25.1 diameter. However, tires are mounted on 8" rims which lowers them somewhat, and measuring the actual height with tire off the car gives closer to 24.1 inches. So ok, use 2.76 and 24 diameter on transmission gear chart and it should be a 30 tooth speedo gear. I have taken the speedo gear out and it is actually a 28 tooth gear. so =30/28d or .93333 or 7% off. multiply that by SHOWN on speedometer 60 and actual speed would be about 56 MPH. BUT my GPS says 48. That is off by 12 mph or 20%.
That being the case to get my speedometer to show the actual 48 I will need a gear with 20% more teeth than the current 28. That would be a 35 toot. But why is this?? Is there something wrong with my calculations?? Or are the speedometers them self that far off on a 42 year old car??
Suggestions??
