Speedometer Driven Gear Help

[murguia]

So I replaced my stock 3 speed with a TH350 and my speedometer is way off. It climbs pretty quickly initially, but when I'm cruising 60/70/80... it tends to be off by about 18-20 mph

I removed the driven gear from my old 3 speed but its much smaller in diameter compared to the one in my new 350. This one is an orange/pink color with 35 teeth.

I tried to do the TCI conversion but got a little confused, so hoping you can all point me in the right direction, so I can buy a new gear to get me as accurate as possible.

235 75 15 tires
3.73 gears
TH350

Thank you

[pjmoreland]

Since you have a 35t driven gear currently, I'm assuming your transmission has the newer large style Speedometer gear housing that works with driven gears in the range of 35-39. I believe 17 driving and 44 driven are what you need. This will require you to get a new housing for 40-45 range driven gears though. A photo of what you've got would be helpful.

https://www.amazon.com/Speedometer-G...931f49b2a36ae5

[murguia]

Originally Posted by pjmoreland

Since you have a 35t driven gear currently, I'm assuming your transmission has the newer large style Speedometer gear housing that works with driven gears in the range of 35-39. I believe 17 driving and 44 driven are what you need. This will require you to get a new housing for 40-45 range driven gears though. A photo of what you've got would be helpful.

https://www.amazon.com/Speedometer-G...931f49b2a36ae5

You are correct. My housing has those same numbers but 36/37…

[pjmoreland]

If you drive exactly 60mph according to GPS on your phone, what speed does your speedometer read?

With this information, we can figure out how many teeth are on your current driving gear, and then we can figure out if you can get away with just replacing the driven gear.

[murguia]

Right around 80

When cruising at 60/70/80/90 on my speedometer, my gps was about 20mph off

[pjmoreland]

Because of the way the math works, we would need the exact speed error at one specific speed. Typically, the error grows worse the faster you go. Is your speedometer reading lower or higher than it should?

[murguia]

It’s reading higher.

When I’m going 60 it says 80

[pjmoreland]

Originally Posted by murguia

It’s reading higher.

When I’m going 60 it says 80

Based on that error, the gears currently in your transmission are sized for 3.73 * 60 / 80 = 2.80 rear end gears. Plugging that into the TCI calculator, it indicates that your speedometer drive gear is an 18t. If that is correct, then you'd need a 47t driven gear to go with your existing drive gear. Problem is they don't make that size. You will need to get a 17t drive gear and a 44t driven gear, along with a 40-45 driven gear housing.

It would be good to double-check the number of teeth on your drive gear. What color is it?

17 = red
18 = blue
19 = yellow
20 = brown

[murguia]

I can’t recall exactly. This is all new to me so I didn’t take a picture of that when I had he housing off. I’ll double check it when I get a moment.


Really appreciate all of your help!