I know someone has the answer so I figured I’d ask. I have a 51 Mercury with a 1971 mustang 302 and C4 trans. What speedo gear do I need to get my stock speedometer to work properly? Thanks!

I did speedometer cals for years at Ford dealers. You need to establish actual speed either on a dyno or using a GPS feature on a cell phone. Ford uses speedometer driven gears ranging from 15 to 21 teeth. Each tooth will change the speedometer reading approximately 2 1/2 mph. If that doesn't fix it, then the output shaft is incorrect. Cars trucks and vans had different shafts because of different tire diameters. Without specifics of your situation other than engine/ trans, that's about as close as I can get.

What is it doing now? My approach is to see if the odometer is reading miles correctly, and also to see if the speedometer is reading speed correctly. If they are both off by the same amount, either fast or slow, I then take out the driven gear and count teeth, and calculate how many more/fewer teeth it needs, based on the percent that the odometer is off. If the necessary gear is not available, then it's time to make a ratio adapter box, or (if it's a GM, not a ford or mopar) I change the driving gear on the output shaft. If the speedometer and odometer are off by different percentages, then I take the speedometer assembly out and adjust the spring, either soften it's action so the speedo will read faster, or tighten to make it read slower. But you didn't tell us what the symptoms are, so we can only guess at what you need to do.

get a bunch of different coloured ( colour denotes number of teeth on the gear) and start with one then go up or down with teeth number, the less teeth the quicker the speedo will read and vice versa, depending on tyre size you may not get the exact speed but you can get close. go up or down in 1 tooth increments.

6.70 x 20's or 6.70 x 13's [or somewhere in between]????? here is a gear calculator , but you will need some basic input numbers Ford speedo's usually turn 1000 revolutions per mile. http://www.wallaceracing.com/speedo-calculator.php

I don't know the exact rolling circumference of your tyres The best method is to put a chalk mark on the ground and also the tyre, then roll the car 1 x revolution and mark the ground and measure the "rolling circumference" which allows for sidewall flex. but I'll assume it's 86.394 based on a 27.5" diameter [from the internet] That equals 733.27 revolutions per mile. [1 mile =63350" divided by 86.394" = 733.268 revolutions] The math formula is [Drive gear number x rear end ratio x tyre revolutions per mile] divided by 1001 = Driven Gear Now the Ford C4 has 3 x drive gears [on the tailshaft] and approx 16 driven gears. Unless I know the actual number of the drive gear I can only guess So I'll do the calculations for each drive gear. [which is 6 tooth, 7 tooth, and 8 tooth] With a 6 tooth drive gear , you will need a 17.23 driven gear [17 tooth is nearest] With a 7 tooth drive gear , you will need a 20.1 driven gear [20 tooth is nearest] With a 8 tooth drive gear , you will need a 22.97 driven gear [23 tooth is nearest] You could buy 3 x driven gears, or pull the tailshaft an count the teeth Attached is the correct way to count the drive gear teeth! Shining a flashlight down the hole and counting across the gear is incorrect. I hope this helps.....

I'll go through the math and all those numbers when I set up a speedo gear in a new build, but normally I just see how far off the odo is and see what the driven gear is now, and get one that will correct the odometer. Saves a lot of hassle with numbers, and getting wrong parts because things aren't quite what you thought they were.

A whole mess of cars built for the US market have speedometers that are built to the standard of 1000 revolutions per mile. To the best of my knowledge, both your chassis, and the donor car were built to this standard. What that means is that if you hook up a cable, you will be in correctable range. Hook up a cable. Download a GPS speedometer app, and see how far off it is, report back the shown speed, and the actual speed, at a couple of whole number actual speeds. Then we can tell you what gear to swap in to fix it. We'll need to know what your driven gear has for teeth currently.