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I have looked at multiple sites. I have a 90 F150 with the M5r2 trans. I just swapped to 4.10 gears and have new, stock 235/75/r15 (28.9”) tires. Every calculator I found says that the correct driven gear would be 20 tooth. Well, that’s WAY OFF! I am correct that the driven gear is 7 tooth for the M5 trans? I bought a 20 tooth and at 45, I’m actually doing around 36-37.
Last edited by 90project5.0; Nov 2, 2025 at 06:30 AM.
The drive gear will have the less teeth and be on the outpot shaft of the trans or transfer case
The driven gear is on the cable end and has the large number of teeth
You probably need to switch the gear inside the trans on the output shaft to a 6 tooth and then experiment with different driven gears ranging from 13 to 20 or so teeth
I’ve seen them, thanks. Is it easy to change the drive gear? A buddy had a 23 tooth laying around and let me use it. It’s a lot closer, but still not there. I corrected the mistake also, thanks!
Last edited by 90project5.0; Nov 2, 2025 at 06:30 AM.
No, not easy to change the drive gear
Not real hard either
Rear output housing off for starters
Some are easier than others.
If it's a 2 wheel drive trans with no transfer case, that is easier than some transfer cases IMO
Some are held with a little steel ball and some with a little clip, be careful either way not to lose the ball or destroy the clip
Sold! I can deal with close enough haha! I barely ever see hwy or interstate driving where I would worry about speed traps. With the 23 tooth driven gear, 45 on speedo is showing 43 on gps.
Is this the chart you used? I went from 3.55 with a yellow driven gear to 4.10 gears. I’d have to go pull the sensor to make sure of the color but my speedometer is correct.
Last edited by My4Fordtrucks; Nov 2, 2025 at 09:29 AM.
Reason: Ugh. Forgot to add the chart
I saw that chart, but for a 7 tooth drive gear, it didn’t show driven tooth for 4.10s. I did use that equation which said 20. I used two different websites, TCI and another (5speeds maybe). Both said 20 tooth. That’s why I was wondering if I had that the M5r2 used 7 tooth drive gear like I found was incorrect. Truck was factory 3.08.
Did you look inside and count the number of teeth on the drive gear to confirm what you have? I have 6 teeth in the transfer case and am using a 21 tooth driven gear.
I have not, I’ll do that next. I never saw anywhere that said different to the 7 tooth for the M5! Now, I know. Thanks for the assistance to both of you!
Sold! I can deal with close enough haha! I barely ever see hwy or interstate driving where I would worry about speed traps. With the 23 tooth driven gear, 45 on speedo is showing 43 on gps.
We used to have to use mile markers on the freeway to test speedos before GPS or cell phones
One tenth was 1 tooth or 6 mph
If you were off by less than 6 mph, changing the driven gear 1 tooth would put your speedo off the other way
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