I recently purchased a cleaned and lubricated speedometer for my '48 GMC 1Ton Dually flatbed using 7.00x18 tires. A drive around my looping street clocked at 1.3 miles, matched by the reading in a different vehicle. The indicated speed is way off, reading about 12.5 mph when the correct speed, observed by following a helper in the other car, was about 30. An indicated 19 on my gauge was an observed 40 mph. Both the odometer and the speedometer functioned smoothly.
Since the rear end and transmission are the same as the '48 Chevy, HO72 and 4 speed, I went to the 1929-1954 Chevrolet Master Parts Catalogue for some information. It shows the 1 Tons using a particular speedometer, with some of the 17" wheel equipped trucks using an adapter for that speedo. The same # speedo is also used in many of the 1-1/2 trucks as well.
My questions are, how can the indicated speed be so far off while the indicated odometer seems to be accurate? Is there an adjustment I can make to get the speed reading accurate? I'm a newbie to the world of speedometers but my understanding of how they function is that there is a spring that restricts the magnetically induced rotation of the needle. Could the spring be the problem? Is there something else that could slow the rotation of the needle?
As always, thanks for the education! Jim