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speedo help
I run a 23 tooth gear in my 70 mercury cyclone it has 3.50 gears and 25.9" tires. When its time to replace them I will go to a taller tire, but after 1500 miles the gear looks brand new. I don't know how long it will last but I going check it when ever I change oil and replace if needed.

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Cool I think I’ll try a 23 also. Maybe they are a better quality now.

run_horse Run Horse Run!
John 72 Q Code
i figured out where i screwed up, i used the rim diameter not the tire diameter, 21 should do me.
I bought an gear box that screws into the transmission and the speedometer cable screws into that.  It was a little challenging to fit in the space. I think I also had to buy a new cable with the correct end. Works perfectly.


73 ragtop, 1999 Mustang Bright Atlantic Blue Paint, Phoenix Engine 302-335HP,  Edelbrock Carb & Performer manifold; c4 with 2000 stall and shiftkit; 3:55 auburn limited slip differential, Hedman shorties; Car Chemistry Exhaust

Classic Air; Tilt Steering Wheel; 1999 Chrysler Sebring bucket Seats ; power windows;
Hi to all,

Just wanted to follow up and pass on my own personal info on my selection of speedo gearing.

OK - My '73 Mustang 302W  vert runs a factory C4 trans with an 8 tooth drive gear. (verified)
My rear tires are 295/50 on a 15 inch rim. That gives 26.6 inch diameter of infated tire under no weight load.
My diff has just been upgraded to Tru Trac Posi with a 3.25 ratio. (the old factory ratio was a 2.75 i believe)

That new combo has thrown my mph speedo reading out close to 6mph too fast. The original factory driven gear was an 18 tooth.
Based on the Internet speedo gearing calculators, they told me a 20 tooth driven gear would be just right to bring my speedo
back to an accurate reading. I bought a 20 tooth driven gear, installed it, and it has proved to be giving me now an accurate reading again
based on many Sat Nav tests conducted by myself.Very happy with my results.This setup has worked for me.

So for all you guys with a similar setup to mine specs wise, this will work for you also.

Many thanks,

Greg. Smile

ok so i ordered what should have been the correct 21 tooth gear, then I swapped it out with the one that was in it, more on that in a second. I tried it out and the speedo just kinda bounces and then floats around over 40. The one I pulled out was green and the center shaft on it was longer, much longer. So I figured eh I got the wrong one, so I went looking for another one from another shop and manufacturer, it looked like the shaft was longer but once it arrived its the same. What gives? The trans is a c6 auto, and the gears I've gotten say they are correct. So is there something I'm missing or do I need to look further down the line to the cable and connectors.
I think the green one is for a manual transmission. The shop where I had mine done said that would work. So the ones for a manual trans will work in the auto. Mine kinda does the same. Might need to be recalibrated?

run_horse Run Horse Run!
John 72 Q Code
As long as the shaft on the end of the speedometer driven gear inserts into the pilot hole (past the drive gear) it should operate properly. Are you sure it was inserted into the transmission all of the way?

If the gear is in properly and the cable housing firmly connected the problem is likely your cable and/or cable housing.

I believe the longer shaft was to make it easier to get the gear into place on the assembly line and the shorter shaft is used on replacement gears.

The green driven gear is for automatic transmissions.

“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
I had a similar issue on a cable driven speedo several years ago on a different car. Tried new gear etc. same issue bounce and just float around. I ended up pulling the cable apart and being from 1962 there was a lot of built up crap. I cleaned it and lubed with graphite and reassembled. Worked flawlessly after that. You might also want to make sure there are not extremely sharp bends in the speedo cable as this will also bind it up and cause this.

73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

- Jason

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