You need to know how far off it is at 40, 50, 60, etc.... Just one speed is not enough information to accurately assess the problem.
It best to drive on marked highway for 10 miles, or use GPS for the same 10 miles.
The odometer is geared and takes all the magnets out of the equation. Once you see how far off the odometer is, then you can adjust gears up or down. If the odometer is slow, put a smaller pinion gear in, if its fast put a bigger gear in.
The speedometer is driven off a rotating magnet, if the magnet gets weak, the needle will read slow, but usually at higher speeds. It's not a linear error, they will read closer to right the slower you are going. It can also be dirty or gummy which makes them read wrong. Has it been cleaned?
As an example, mine was slow at 60 mph, but right at 30 mph. The magnet is weak, at 70 mph it was even farther off. To correct it, I pulled the needle off and moved it forward, I want it to be right at 60 mph, this is where I do most of my driving. At slower speeds it now reads fast so if I use like I should, I should never get pulled over for speeding at slower speeds.
1000 rpm at the speedometer cable equals a 60 mph reading, this is the US standard calibration number. I used an electric drill I calibrated to 1000 rpm ( hand held digital tachometer ) clamped in a vise with a short speedometer cable attached to my speedometer. It was easy to adjust while on the bench. The key is having a calibrated rpm to work with.
Found this on the web:
Calibrating a Mechanical Cable-driven Speedometer
One of these methods can be used to determine whether your speedometer is accurate or needs to be recalibrated.
Method-1:
A–Find a measured mile. Use the mile markers on an interstate highway or a speedometer test zone. Interstate highways have markers for each mile and some state or county roads have markers that can be used if you pay close attention to the tenth-of a-mile markers.
B–Check the odometer reading, in miles and tenths of a mile, as you pass a mile marker and then check it again as you pass the next mile marker.
C–Check it again two more times to be sure that your reading is accurate.
Method-2:
A–Unscrew cable end at the speedometer head.
B–Attach a flag on the end of the flexible cable formed from a piece of tape or by using a paper clip.
C–Measure a distance of 52-feet 9-1/2-inches and mark at each end with chalk or a piece of tape.
D–Roll the car up to the first mark making sure that the cable has begun to turn. You will probably need someone to help by keeping track of the cable turning.
E–Put a piece of tape on the side of the tire to align with the first mark. Push or drive from the first mark to the second mark.
F–Count and record the number of full turns and tenths of a turn that the cable makes. Ten full turns is correct for domestic cars. Note how many tenths of a turn, either over or under ten full turns, that the cable makes.
G–Repeat the test two more times to verify the accuracy of the results. From those results a speedometer specialist can build a ratio adapter for the vehicle.
Last edited by Joe H; 07/24/2021 1:19 PM.
