Hi,

I have a 1970 with an automatic THM400, 3.08 gear ratio, tires are P255/60R15... I searched several threads but didn't see anything that answered my question... my car's speedometer is running about 6mph less than what is showing... so the speedometer shows 30mph, but I'm actually going around 24 mph... I understand that I can change the driven gear to a different one and if I am understanding correctly, I would have to get one with more teeth to decrease the speedometer speed showing (but I might be getting that backwards). I've also heard that an increase/decrease of 1 tooth would be a difference of 3mph, but I also heard 5mph... not sure if there's an exact science or not.

First I hope that I am getting the terminology right... my interpretation is that the driven gear is the one on the end of the speedo cable that is located in the housing affixed to the transmission, and the drive gear is the one inside the transmission on the shaft...

So my main question is that if I change the driven gear (the one on the end of the speedo cable), does the internal gear have to change as well? Friend of mine is under the impression if I change the external driven gear, I also need to change the internal drive gear.

Thanks.

 

you will need to install a driven gear with more teeth. the more teeth on the driven gear the slower your speedo will display one tooth equals about 1.5 mph difference. my th400 has a drive gear with 15 teeth and i have about 6 different driven gears to adjust for different gear ratios and tire diameters. with the 15 tooth drive gear each driven gear tooth is 1.5mph difference. the driven gear with the most teeth available is 45 teeth. if your correct driven gear equates to needing more than 45 teeth then you will need to change the drive gear on the output shaft of the trans (would need to drop down a tooth). as far as drive gears i believe there are about 3 or 4 gears available something like 15,16.18 tooth gears. check TCI's website they list and sell most of the gears for the th400. they also have calculators where you can plug in your info - tire diameter,rear gear etc and it will tell you what driven gear you need. on my set up i have a 15 tooth drive gear and 255/60-15 rear tires with a 3.70 differential. my driven gear has 41 teeth to make my speedo accurate the driven gears are also available from many of the corvette vendors.

 

Yep, your terminology is correct. And no, one gear can be changed without changing the other.

Is the 3.08 rear end original? Does your car have its original speedo gears? With your setup the driven gear would be 1362049, green with 42 teeth. I don't know what sort of difference to expect by changing gears.

Here are the originally available driven gears:

 

Also be careful, for a given trans internal worm gear (drive gear) you can only use a certain number of driven gears. If your driven gear gets to big it can bind against the worm gear, and visa versa, the driven gear might not be big enough to mesh into the worn gear. The drive gear has something like 15, 16, 17, 18 teeth. each paired with a driven gear. The driven gear is easy to change. The drive gear isn't. You would need to take the trans apart.

 

never heard of this before however it should be noted that there are 2 different driven gear housings to use. one housing is for gears 35-39 and the other is for 40-45. it's possible that using the incorrect housing would cause the problem you are describing above

 

Thanks guys... this helps a lot... I can't say for sure that my diff is actually a 3.08, so I will need to count the number of teeth to find out for sure... and I haven't removed the driven gear yet, so I'm not sure which one is installed...

But this should give me the information I need to correct the speedometer...

 

you should be able to get the code off the bottom of the carrier and look up what gears are in the diff. assuming they are original.

 

Just drive 6mph "faster"!!!!!!!

 

What does your speedometer show when you are actually going 48 mph? 60 mph?

You can change the drive ratio at the transmission. But it's a ratio, not a flat offset. If you have a 6 mph offset at all speeds, something else is going on.

 

I have a similar issue with my 68, but I attribute it to the smaller 60 series tires that I’m running. There is a calculator on the Discount Tire website where you put in your original tire size and what you’re currently running and it’ll show the difference between what your speedometer is showing and your actual speed.

 

Here are the 1970 differential codes and rear axle ratios...

 

If your speedo reads correct at only one speed, then reads higher at higher speeds and lower at lower speeds, a different gear will just move the speed where it is correct. If the speedo is constantly high or low, a different gear will fix it. Yes,,,, check your gear housing for the correct gear sizes. If your at the limit of gears sizes you might need a different housing and different drive gear.

 

If it is 6 mph 'slow' at 30 mph, it will likely be 12 mph 'slow' at 60. IF (note the bold, italic & underlining on the word "if") the speedo drive gear (gear mounted on the trans output shaft) can accept it, the speedo driven gear needs to have 4 fewer teeth on it than the gear you have now. When you pull the driven gear out of the speedo drive housing (left rear of the transmission), look down inside that hole and determine the color of the drive gear. The speedo gear table for the THM-400 will tell you the number of teeth on the drive gear AND the acceptable range in teeth that can be on the driven gear matched with it. If a driven gear with 4 fewer teeth is acceptable, buy one and replace the one you have. It may be 2-3 mph "fast" or "slow" of actual speed; but it will be a LOT closer than what you have now.

 

drive gear 1 tooth is 10 mph

driven gear 1 tooth is 2 mph

The housing rotates for fitment so it will fit any gear GM TURBO TH400 TRANSMISSION SPEEDOMETER GEAR KIT 15/40 41 42 43 44 45 W/HOUSING | eBay

 

i think you have this backwards. he would need 4 more teeth on the driven gear. he needs to slow down the needle in his speedo to match his actual speed. he said when he is doing 24mph actual his speedo reads 30mph. and if he was off by 12 at 60 he would need 6-7 more teeth on the driven gear.

 

You are correct. I read it the other way (by mistake). Since he is going faster than the speedo shows, he needs 4-5 MORE teeth to slow the rotations down.

Thanks for the correction.

 

the housing can not rotate as it's clamped/clocked into position by the "Y" shaped piece of flat metal

there are two housings available (bottom of page), 34-39 & 40-45