Hello Gang.
I know there is a sticky about this subject and I have read it. I have been on websites with gearing calculators. I am at a loss.

I bought my 82 TA a little over a month ago. I had a TH350 swapped in before the previous owner along with another rear with 3.73 gearing. (It really wails going down the highway) My tire size is 235/60-15. The speedometer didn't work when I bought it. The guy I bought it from said the speedo was never hooked up.

Last week I looked into it. Everything was hooked up. After watching Youtube videos and inspecting the cable I suspected that the cable along with the "bullet" and/or gear was where the problem was. I got new ones and installed them last night. The gear that was in the transmission was the white one (19 tooth) so I replaced it with my new one along with the new cable. Took it for a test drive and the speedo worked. BUT, the speed was way off. At a speed around 35-40 mph (guessing) the speedo was showing around 60 mph. I did some verifying with my GPS today. At 35 mph it was showing 56 mph on the speedo. The speedo was buried when my actual speed was 65 mph. Using a vehicle speed calculator I verified the 3.73 rear end gearing with a 3100rpm engine speed at 65 mph.

Now, I'm not sure what the driver gear is in the transmission. It's cramped and hard to see but I did take a mirror and a flashlight to look in and it appears to be a pink gear (7 tooth if it's a GM piece). I can't count the teeth so I don't know for sure which one it really has.

I replaced the white gear with the gray gear (22 teeth) and went for another ride. I didn't verify with the GPS this time but at the 35 - 40 mph (guesstimated) the speedo showed around 45 mph. It brought it down but not far enough. It looks like the biggest gear I can get is a 25 tooth gear (orange).

Where I'm baffled is the gear ratio calculator that I'm using (https://www.5speeds.com/calculators/SpeedoCalc.html) shows that with a 7 tooth drive gear my driven gear should be a 20 tooth driven gear. The 19 tooth gear should have been close if that's the case but I'm closer to being correct with the higher tooth count.

Does anyone have a recommendation?

 

Before doing anything else that takes effort, is the odometer correct over a drive past a bunch of highway milemarkers? If it is, your speedo may need a recal.
I have used a 1:3 ratio of speedo gears for TH350, 200-4R, 700-R4, and T5 with that tire size. Because GM did with the 215-65R15.
Every time, the odo. would read closer to 30.0 than to 30.1 or 29.9 when I'd check function over 30mi.

I don't go by color, but my wild guess from the peanut gallery is you have an 8t drive gear. Or the speedo reading may be off. There's no way for a properly functioning speedo and a 7t drive to need a 24 driven with that tire size.

 

jmd is right, 7 & 20, 21, or 22 is most likely to be the right combo for your gears. In NO case should it be as far off as you report.

Something else is wrong.

I also agree with him about checking with the odometer instead of the speedo. It's "simpler" in a way. All it does is count how many times the cable spins. 1000 turns of the cable = 1 mile. Incidentally, not 1001, as you sometimes hear; THIMK, a gear train that divides 1 mile into tenths means that the next part of the train would have to divide by 100.1, and that's not possible, because you can't very well have tenths of a gear tooth... what REALLY happens is, the odo goes to the next mile at the start of the 1001st spin of the cable. Just like a new century doesn't begin the 1st of the year ending in 00, since there's no year 0. Anyway, the odo will tell you if your whole gear train is right, which is to say, that your speedo gears set the cable to rotate at the rate of 1000 turns per mile. The speedo has an additional way it can read wrong (well, several actually) and therefore can have other errors that are NOT the speedo gears. The odo CANNOT. Its measurement method is as basic as it can possibly get. If the odo is right, the speedo gears are also right; if the gears are wrong, the odo will be wrong. Period.

Just drive a marked section of freeway for 10 miles. That'll be far enough to be accurate enough to tell. If your speedo gears are right, you'll be within about 2½% (a quarter mile) on your odo over a 10-mile measured stretch. The reason I say within about 2½% is, you CAN'T get any more precise than that; the minimum step you can take in your speedo gears is 1/20, 1/21, 1/22, etc., which means the smallest correction you can possibly make, is a bit under 5%.

Once you verify that the odo is correct, then if the speedo isn't, then it will be time to either fix it (not likely) or just get another.

Start by MEASURING your tire radius, by PULLING A MEASURING TAPE ON ONE from the center of the wheel to the earth, on a level surface, after checking the tire pressure when cold and making sure it's exactly right in both rears, and then driving the car for awhile (tire pressure changes when driving, and it's pointless to dial in your speedo for when the car is cold). DO NOT count on the "trade" size stamped on the sidewall!!!! MEASURE MEASURE MEASURE MEASURE MEASURE MEASURE.

Let's do an example, and pluck a number out of my a... thin air... and say you come up with 13" as your tire radius (distance from center of wheel to the ground), within 1/8" or so. (pointless to get much more precise aka "accurate" than 1/8" in this; the tire size changes more than that just from tread wear old to new, or from winter to summer, or cold to hot, or over acoupla months of normal slow pressure loss... resist the temptation to get into "measure with micrometer, mark with chalk, cut with axe" paralysis by analysis) Then:

• 13" × 2 × pi ÷ 12 = 6.807' (how many feet a tire travels in one revolution)
• 5280' ÷ 6.807' = 775.7 (how many times the tire rolls in one mile)
• 775.7 × 41 ÷ 11 = 2891.2 (how many times the drive shaft spins in one mile, given your rear gears which are 41 & 11, aka 3.73)
• 2891.2 ÷ 1000 = 2.8912 (the speedo gear ratio you need to make the cable spin exactly 1000 revolutions in one mile)
• 2.8912 × 7 = 20.24 (the driven gear you need to get this ratio with a 7-tooth drive gear)

You can't buy a gear like that, with 20.24 teeth, but you CAN buy a 20. With 7 & 20 your odometer will be a tweeek over 1% off, and will read high, since the drive shaft will be spinning slightly more than 1000 times per mile; specifically, your odo will read a shade over 10.1 on a 10-mile stretch.

MEASURE your tires, perform the calculations as above, install the driven gear as described, MAKING SURE you have a 7-tooth drive gear. It should be a sort of purplish blue; if it's ANY other color, red white or ANYTHING ELSE, you will have to fix that first. In fact, as cheeeep as those are and as eeeeezy to change as they are in a 350, might as well just buy one of those and a 20, 21, & 22 driven; put your new blue/purple 7 in, and maybe the 21, and then do the MEASURE/calculate/drive/select exercise.

 

This will solve the problem:

https://www.speedhut.com/Speedbox.html

GD

 

SofaKingdom, thanks for all that info. I'll have to look into it more in depth. I was out and about today and thought I'd check out the odometer with how it's set up right now. With the 22 tooth driven gear and not yet truly knowing what drive gear I have my odometer is 3.6 miles over. At 10 miles on the highway my odometer read 13.6 miles.

Hopefully I'll get the chance to look into it more this weekend.

 

OK, so your speedo gears are wrong. Clearly.

For them to be off by 36% tells me that you HAVE TO have the wrong drive gear in it now. Probably a 9-tooth, maybe 10 (if there even IS such a thing).

Buy the 7-tooth drive and an assortment of driven, like 19, 20, & 21, since you already have the 22; and go from there.