I have a 72 skylark that the speedometer was fast about 9-10mph. It had a 2.56 peg leg rear end. I put in a 3.08 posi rear and a TH400.I also installed 295x50x15 with a 26.7" height tires. I switched the gear in the tranny to a brown one to match my ratio that was on a site given to me by this board. It is still 9-10 mph fast! I am putting in another Th400 and it has an 11 tooth gear. What gear should I buy for the speedometer? The case will accept 37-38-39-40. Thanks Tim
Here is an excellent calculator page from TCI. It's all you'll need to figure it all out. http://www.tciauto.com/tech_info/speedo_gears.htm
Tim, Did you ever count the teeth on the speedo drive gear? That's the only piece of info that is missing from your post. It's all you need to figure out what driven gear you need. You may need to change the sleeve if you need a 40 tooth or higher gear. Just plug in the numbers to the calculator (Phil's link above)
Larry, the gear has 11 teeth which comes out to a 2.33 calculation, however I can,t find the number that makes sense with an 11 tooth gear????? Tim
Tim, Is it possible that you counted wrong. The governor gear has 11 teeth, you aren't looking at that are you? Take out the sleeve, the drive gear is usually metal, and has 16-18 teeth. It is located on the output shaft of the transmission. With an 11 tooth drive gear, you would need a 25 or 26 tooth driven gear. Never seen a THM 400 with that combo. I think there is a mistake somewhere.
You were right Larry, I have a 17 tooth drive gear which doesn't help me too much since my 2.33 ratio calls for an 18 and 42 or a 15 and 35 gear combo. My speedo housing lists 36-37-38-39 tooth gear so now what??? Tim
They make another housing which goes into the higher numbers. Go to any good transmission shop and they should have one. Shouldn't cost you more than $10. You could probably get the other gear to match also. That's what I did when I was looking. Shop had a box of them. Just had to dig around for a few minutes and found everything I needed, for $5.00. Using your current combo, if you get a 41 tooth, your speedometer will read a little high, maybe 2-3 mph at 60 mph. If you get a 42 tooth, you'll read a little low, @2-3 lower.
Thanks Phil, am I just getting the small housing with the numbers on it that the speedo gear fits into? Tim
Tim, You have a 3.08 axle ratio, 17 tooth drive gear, and 26.7" tall tires. That's all you need. Plug those numbers into the formula. 20168 divided by 26.7 =755.36 17 X 3.08 X 755.36, divided by 1001 = 39.51 That means you can use either the 39 tooth gear, or the 40 tooth gear. 39 tooth gear (brown) is GM part # 1359273, and it will fit in your housing.
Larry, I have a 39 gear and my speedometer is 9-10 mph fast! It was alos 9-10 mph fast when everything was stock too! Now what? Thanks Tim
Hi, get the 40-45 tooth housing, I swear to you a good transmission, possibly even an AAMCO, will have a box of them along with gears. The little bolt and tab is all that holds them in. They slide in and out. You'll probably be able to get the gears from him also. My guess being that far off you'll need a 41 or 42 tooth gear. See if the tranny shop has all 6 and promise them you'll bring back the 5 you don't need. If you can't find one, I may have one lying around here.
Thanks Phillip, why do you suppose that the speed is so far off even when I use the gear that the I am supposed to use. I will go see some tranny shops today. Tim
Phil, I picked up a housing fro $5.00 at a TSI shop today! Thanks for the tip! I spoke with the tech there and he said that I will probably need to change the drive gear also to make up for the 9-mph difference. I ordered a 43 tooth gear anyway and I hope it's close. What do you think it will take to make up the difference? Tim
Sorry but I can't help you with that one. You've changed so many things between rearend, tires, no both gears. It'll be a best guess from here on out and trial&error to get it right. But I know from experience that it feels so good when it's finally right.
Tim, How do you know it's 9-10 MPH fast? Are you sure of all your numbers? Your tires are closer to 26.6" tall. Redoing the math, the answer comes closer to 40 teeth. I would try the 40 tooth gear (black) 1362048. The sleeve for gears 40-41-42-43-44 is 1362293
Wait a minute, did you say a 9 tooth drive gear? I thought this was for a th400? That gear is shown for a th350, but not for a th400.
I have a 17 tooth drive gear. I was on the freeway next to a buddy driving my Aurora at a steady 70mph, my Buick was at 79-80mph with the 39 gear!!! I will try the 43 and see what happens! I will keep you posted! thanks Tim
Tim, If you have a tach in the car, 60 MPH with a 3.08 rear and 26.6" tires should be between 2300 and 2400 RPM.
Here's another application. How about a (Jim Burek) 200r4 with 3.73 rear end and 235x60 tires? Right now I think it's reading about 10-15 mph high (about 80) at 65. Is this a pretty easy fix on a 200r4 also?
Same procedure. I'm doing mine right now and I think I need to swap the 31 gear with a 30. The biggest thing is finding out which drive gear the trans currently has. I just had my trans rebuilt and know it's an 11 tooth gear which was factory for the GN's.
