How do I calculate how many teeth on a speedo gear I need to make the speedometer read correctly? The rear end was changed in my car before I bought it and the speedometer reads about 10mph high. I need a new speedometer cable o-ring anyway, so I might as well buy a new gear.

3.33's in the rear, but how do I figure out what gear I need?

 

Give this a try. May have to change both the drive and driven gear to get it right. Mine was off a bunch and used a ratio adaptor instead.

http://www.tciauto.com/tc/speedometer-gear-calculator/

 

With that much of a difference you will likely have to change the drive gear on the trans tail shaft. It can be done in the car (email if you want detail).
The driven gears are made in ranges(tooth count) that correspond with a certain drive gear. Driven gears as you know are color coded to indicate range which correspond to the tooth count on the drive gear. These sets are available through several sources ATI, TCI, Hughes, B&M etc...or a speedometer restorer, I've even lucked out at a local trans shop. Chances of a GM dealer having them today is slim.
Your other alternative is the adapters hookem mentioned. I haven't had good luck with the adapters. I cant find one made in the USA that last longer than a month or leak but this may be the direction you want to go, less labor intensive. The driven gear removal and installation should be handled by a pro.

 

http://www.transmissioncenter.net/sp...n_______va.htm

Good info here too.

 

The speedo gear housing should have some numbers on the side of it. This will let you know the driven gears that will fit inside the housing (like 36, 37, 38, 39). You may have to remove the tailstock to see the side of the drive gear as sometimes it is visible from the end. Mine in my TH350 were. I had a 18 tooth drive gear and put a 42 tooth in its place when I changed the rear from 2:73 to 3:08. I also had to drill the housing as the 42 tooth gear shaft is a little larger in diameter and I didn't find the 40, 41, 42, 43 tooth housing. You will need to know the drive gear to get the applicable driven one. Try www.transmissioncenter.org/GM_Speedometer_Gears.

 

What tire size are you running? This will also play a role in the calibration. I have the gear tables in my Olds shop manual and can tell you what you need if you tell me what tire you are running.

 

what adapter did you use? Does it leak? Ever had any problems with it?
Great... :/ Thanks for sharing your experience.

They are either 235 or 225/70/R14's. The car originally had 2.73 gears in the rear. What page is the chart on? I looked through the manual on wild about cars but didn't see it.

Hoping I don't have to change a lot of stuff. I've been told it's really easy to damage the transmission even just replacing the O ring for the speedo gear. Apparently there is a special tool, and one should not use an awl.

I may end up bringing it to a local transmission shop, but I want all the parts and pieces.

 

what trans do you have?...with your new gears your going to need one but you shouldnt have an adapter with the 2.73's

 

I ordered the adaptor from transmissioncenter referenced above. Previous owner had upgraded the stock jetaway and rearend to a TH350 and 3:42 posi (nice!) but that left the speedo off by ~25%. Would have needed to swap drive & driven gears so I just ordered the down 26.7% adaptor. Took 15 minutes to install with no leaks or problems in over a year of weekend type use. Now about 5 mph low on the highway with some big 275/60R/15's on Cragars and really close with 225/70R/14's on SSI's. IMO, good solution if you cannot get away with changing the driven gear only.

 

Th-350 trans. I don't currently have an adapter. The car somewhere in it's life had the rear end gears changed from 2.73's to 3.33's. Trying to re-calibrate speedo for 3.33.

Thanks for the input. I may end up going that route too.

 

JPC

did you end up repairing this. I just went from 2:7x open rear end to 3:42s and now I need to adjust the speedo.

To anyone how can I tell how much the speedo is off now, it appeared to be DB accurate before the rear end change (at least at 30 mph +-)

from the manual it looks like i have a 39t in there and no need a 45t (light blue) is that the gear that requires a pro to change? if so maybe the ratio adjuster is right for me ? comments fron anyone who has done this

 

I didn't change it yet. I've been otherwise occupied on my intake/carb/timing adjustments.

I don't think a "pro" is needed, but a special tool is to replace the seal. Using an awl could scratch the surface and no seal will ever seal. A local transmission place could probably do it in 25 minutes, if you brought them a new seal and a new gear. I never quite figured out which one I need. Where in the manual did you see the gears?

 

Pg 4b-23of the 70 CSM has a speedo gear ref chart....I might go w the adapter looks like it would be the easier route....I gotta figure my error percentage first

 

Wait you changed youre drive gear? Couldn't you have just changed your driven gear from the 2.73 to the 3.08(like a 38 tooth to a 45 tooth).

You took the drive gear out the tail and put a 42 tooth driven gear in it's place?


unfortunately my car/manual are 1972 and I can't seem to find this chart in the manual. But I found it. Page 4B-37.

Now I need the tire size chart. I think the SS2 wheels were g70, but I don't want to guess wrong.


If olds only offered a 3.23 rear and a 3.42 rear, and I have a 3.33 rear, which would I be better off choosing a gear for? The chart doesn't like a 3.42 available with the th-350.

 

bump. Not sure what to buy... :/

Having a gear that wasn't even offered doesn't help. I remember checking the teeth on the rear end a couple years back and it came out to 3.33's. An adapter might be my only option, but not to figure out how far, percentage wise I'm off. Guess I'll be doing a run behind another car or two to check the mph..

 

Find your percentage error. I think most highway mile markers are pretty accurate. The farther you go, the better. You're reading fast so lets say that for 5 measured miles, your odometer shows 5.9 mi. 5.9 divided by 5 = 1.18. (18% fast) Now this next part isn't always do-able. Take the number of teeth on your driven gear and multiply it by 1.18, Assuming 18 teeth, (18 x 1.18 = 21.24) a 21 tooth gear would get you very close (better a little fast than slow). Now the problem is, the particular tooth count may not be available or it may not fit your sleeve. But this will give you an idea of what you need to change, and if lucky, a cheaper, easier way out.

 

Use a GPS (stand alone or freebie phone ap) to get real MPH, then figure out percentage error

 

Isn't it the Drive gear that would have 18 teeth, and the driven gear that's got 36+?

The chart I looked at showed drive gears 18 teeth for th350 trannys?

Good idea. Thanks. Usually they are pretty close, within a mile or two..

 

PATC is telling me I need more information..

I told them the rear end was originally a 2.73 matched up to the th350, and that the stock chart shows a drive gear of 18teeth and a driven of 38.

Now the car has 3.23's in the rear, and I'm thinking the driven gear needs to be 45 teeth, but then he asked about a "drab" or "drav" gear, that either has 7, 8, 9, or 10 teeth on it. WTH is this? He said it's connected to the driven gear.

I'm confused now.

 

The numbers in my example were only meant to show how the math worked. That's why I said "assuming 18 teeth" because I don't really know what you're working with. Wasn't till post #10 that we see it's a THM350. THM350 driven gears start at about 18T (with drive gears of around 8 or 9T). Take the number of teeth on your driven gear and plug it into the equation and see if it gives you an available tooth count.

 

Okay thanks. I understand the math. Is the following chart wrong:
speedochartmarked_zps77437540.jpg

To me it looks like I have a 18T Drive Gear, and currently either a 37/38T driven gear(depending on original tire size, which btw, how can I determine what It came with).

All the TH-350's have an 18T Drive Gear, according to this 1972 Chart,so I need a 44/45T Driven Gear. Correct?

The driven gear is the one connected to the speedometer, correct? What is this gear with 8 or 9T you refer to?

 

The chart is not wrong. I didn't know what year, make or model you were working on. SOME THM350's came with 8T drive gears (mid to late 70's pickups, one example). Yes, the driven gear is the one on the end of the cable. A 44 or 45T gear should get you close. but look at the numbers on your sleeve to be sure it will fit (see post# 5). How many teeth do you have now?

 

I believe I currently would have a 38 tooth gear. Assuming it's original. I've had an hard time getting the connector off the speedometer cable, so I haven't pulled it to check. It's a round screw on type thing, but I can't get it off. Not with vice grips, not with anything, yet.

I may have to replace it with the speedometer bullet thing, if I break it.

 

It appears that Oldsmobiles turbo 350 and 400 came with 18t gear in tail shaft. So if I pull the speedometer cable and gear I should be able to see what drive gear is installed by the color?

 

"It appears that Oldsmobiles turbo 350 and 400 came with 18t gear in tail shaft". Not true. There was a range of gears that could be installed on the tail shafts depending on what gear ratio was in the rear end as depicted by the chart above. No color code on the drive gear its a steel gear. You can mark a tooth with a sharpie and spin the tail shaft and count. If its out of range for your driven gear this gear can be removed while the trans is in the car if needed. Its not easy but I've done it. A good trans shop will have the correct matching set for your application.

 

thnx for the clarification!!! It has been a big help. I pulled the speedo gear last night and it had a 36 tooth gear and the rear that came out was 2.56. Which should calculate to a 18 tooth drive gear. My tires are 235/60r14 Bf Goodrich t/a. The build sheet on my SX shows a 35 tooth gear for a 2.56 rear. The original rear in my 442 was a 3.23 and the build sheet shows a 45 tooth gear so I will order one of those. It appears the person that restored it in 1993 must switch the rear out for some reason.

 

Bumping this one to the top. My '69 A body has a 2.56 rear and I'm running 225/70R14 tires on her. According to tirerack.com the tire would be equal to an OE size of 8.25 X 14. The speedo chart in the fsm for a TH350 lists a drive/driven combination of 18T and 35T. I have a 35T driven gear (35T stamped on it) and a green drive gear. I believe the green gear to be 18T but it isn't labeled. I have a yellow/orange drive gear that's labeled 19T. Can anyone confirm the 18T drive gear is green? Also if I'm a tooth off and need to correct what is the range you can combine with the 18T drive gear? 34T-39T driven?

 

This link indicates that a green driven gear for a TH350 is 9 teeth and an 18 tooth drive gear is blue:


http://www.tciauto.com/tc/speedometer-gear-calculator/


If you have a 19-tooth drive gear (yellow according to the above link), you could use a 37 tooth driven gear (red) to get the same ratio offered by the 18T/35T combination.

 

Using this calculator to determine correct driven gear in '70 TH350: http://www.bgsoflex.com/speedo1.html The calculator requires the number of revolutions of the speedo cable to register 1 mile on the odometer.

How many speedo cable revolutions on a '70 speedometer = 1 mile? Is it 1,000 or some other number? It may be stamped on the speedometer case.

 

1000 rpm = 60 mph is the standard.