Might want to check that rear gear ratio

 

You need to know three things.

How many teeth are cast into your output shaft. I think on a C4 it's usually 7 or 8, but if you don't know for sure you'll have to count them. Hopefully you can pull out the speedometer cable and look in the hole to count them.

Tire height. If you know the numbers on your sidewalls, there are a number of online tire height calculators that you can use.

Finally, you need to know your rear end ratio. You stated 2.33. As Hemikiller said, you might want to check that because 2.33 would be a very unusual rear end ratio. Most factory rear gears run from 2.79 up to 4.11. There are aftermarket gear sets that are higher and lower, but 2.33 would be very unusual.

Once you know all these numbers, there are online calculators that you can use to determine which speedometer gear you need. Summit, cjpp, jegs, etc.

 

Just occurred to me where the 2.33 gear was used - the Mustang "E" model. If that's what you truly have, then the factory driven gear was 15 teeth. If that's not available, get a 16th and you'll be fairly close, if not a little slow - depending on your tires.

 

Mustang E' is exactly what I have. It still has the tag on the rear with 2:33 stamped and is correct for the E'. I took the trans into a local transmission guy. Been doing it for as long as I can remember. The first thing out of his mouth was that I have a very rare transmission. He had only seen one previously like it in his entire career. I suppose he could tell me the correct speedometer gear to get. Thanks all for your replies.

 

If the car operational now use a gps or google maps on your phone gps to tell you your actual speed at about 40-45mph. Note what your speedo indicates. Determine what percentage it is off either high or low. Remove the speedo cable from the tranny and look at the driven gear. There should be a number stamped on the gear such as 17 or 18 indicating number of teeth. If not just count them.


Lets say you have 18 teeth now and you found your that your speedo reads low by 4 mph at about 40-45mph as mine did. So that is about 10% low. With your speedo low, you want fewer teeth (If it had read too high, more teeth). Then, just figure the percentage difference between your 18 tooth gear and ones with (in this example) fewer teeth and choose the closest one.



This is more cumbersome than using a calculator for sure. But, I thought it would be more accurate. If you want to use a calculator, by all means do so. If you can find the revolutions per mile spec of your tires and a calculator that use that spec instead of diameter or circumference, I think that would be the most accurate calculator. I looked but could not find a calculator using the revolutions spec so I did it the hard way. Mine is spot on now.

 

Thank you, I never considered that. Good idea.

 

I did something similar to 1968Cally's method when I put a T5 in my car. If you can find a spot where a measured mile is marked off (I have a road I travel down to work next to some RR tracks that have mile marker posts) then record where the odometer 1/10th mile indication is and see where it is at the end of the measured mile. It doesn't matter what speed you travel at. I found that the odometer indicated 0.9 miles at the end of the measured mile so I counted the teeth on the current speedo gear and ordered one that had 10% less teeth to get to the right gear. It worked great after that.