If your speedometer is mechanical and not electronic this should work to make it accurate.
When I had my ’32 with the Hemi in it, the speedometer was way slow. Here is how I fixed it.
First you have to disconnect the speedometer cable from the back of the speedometer. You need a rev-counter like the one shown in the picture or one similar that will work. You then hook the end of the speedometer cable to the spindle on the rev counter as shown in the picture. I used a small piece of rubber vacuum hose. Disconnect the two for now.
The next thing is to find a good section of highway that has clearly marked mile markers that is safe to stop on. I used Hwy 49 not far from Lime Kiln on a Sunday morning. Pull the car to stop at a mile marker and hook up the rev counter as shown. Make sure the counter is set at O. Drive one mile to the next marker and stop again. Write down the no. of revolutions or disconnect the counter. The important thing here is that the industry standard is 1000 revs. per mile. If your speedometer is slow your number will be less than 1000 or vise versa.
Next you take this info to Commercial Speedometer in West Sac. They will take this number and make up the proper over/underdrive for your car. They did mine while I waited and I took it home and installed it and it worked just fine. I have 2 of these rev counters so if you want to borrow one give me a call and I will bring one to breakfast and you can use it.
– Jack E.