'65 speedo out of whack

[William Dillon]

My speedo seems to be reading about 15 to 20 mph slower than what it should be. My car has 3:70 gears, 205/70-15 tires, a new speedo cable and restored speedo. I'm currently running the "green" gear which was suggested to me, still too slow. Any suggestions? Thanks.

Bill

[Duke W.]

Re: '65 speedo out of whack

At what speed? The data you need is the PERCENT DEVIATION! Take error readings at various speeds between 30 and 80 and compute the percent error. You can use a modern car to pace as a reference. If the error is a constant percentage at all speeds you have a "gain error" which can be corrected with speedo head calibration or gears. If it is a constant speed error at all indicated speeds, it is an offset error which is probably in the head. It's also possible to have a combination of both.

If memory serves the correct driven gear tooth count for a 3.70 axle is 22, but I don't remember the color. However, the correct DRIVE GEAR inside the transmission must be installed, so if the transmission is from a different year/era or for whatever reason has the wrong drive gear, you have a bigger problem.

Even with correct gearing and head calibration your speedo should read about five to six percent high because your tires are shorter than the 760 revs/mile that the speedo system is calibated for.

You can look up your installed tires on the manufacturer's web site and get the revs/mile spec. An approximate five percent error for short tires can be corrected by obtaining a drive gear with one less tooth than OE if it is available.

Duke