72-CLASSIC_RIDE Posted July 20, 2014 Share Posted July 20, 2014 Need some advice trying to determine driven gear teeth count for my tire size and rear axle. I had changed from P235/70/15R (approx. 29 1/2" tall) tires to P245/60/15R (approx. 26.53" tall) tires and found my odometer was reading about 2 tenths more miles traveled per mile when measuring against posted mile marker highway signs. Called TCI who said based on tire size, Transmission and 331:1 rear axle ratio, I needed a 38 tooth driven gear which I purchased along with the housing. Also, was said my drive gear tooth count was 15 teeth and was standard for all TH400 Stock transmissions. Does this seem correct? What was removed (driven gear) of 43 tooth count and 38 tooth count gear added. While I had not as yet been able to determine that difference in speedo or odometer accuracy, based on previous experience (tach reading), I know this 38 tooth gear added is not correct as it suggests turning to fast for actual speed. So, what I am seeking is: TH400 Tranny = 15 tooth count drive gear (Stock transmission) Rear Tires = P245/60/15R @ 758.92 KM (471.57) revolutions per mile 331:1 Posi-Traction Rear Axle When attempting to use TCI's calculator, there is no calculation available for the 331:1 rear axle? And clearly, the one they recommended is no where near what is needed. With the TH400 tranny, the largest driven gear size is 45 teeth with the one I removed a 43 tooth count and replaced recently with a 38 tooth count. I'm guessing that that tooth count might be closer to the 45 tooth driven gear as again guessing it would slow down the speedo/odometer readings. As I am not comfortable or knowledgeable in determining what driven gear tooth count I need, am requesting your advise/help in finding the closest driven gear needed to correct speedo/odometer accuracy. Your help appreciated as always. Doug Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted July 20, 2014 Share Posted July 20, 2014 first you need to verify what drive gear you have and are you positive you have a 3.31 axle ratio? which way off was your speedo with the 43 tooth gear? I used a different companies calculator and with a 15 tooth drive gear, it also said a 38 tooth driven gear http://www.bgsoflex.com/speedo1.html Quote Link to comment Share on other sites More sharing options...
stangeba Posted July 20, 2014 Share Posted July 20, 2014 Borrow a Garman and see how close the MPH is but I also came up with the same gear they did. Unless you are the original owner and know everything that was ever done to the car, you are just guessing. Verify the axle ratio and not just by checking the tag, rotate the tires both one turn and carefully count the drive shaft distance. Input Parameters Are the Following: Speedometer Revolutions/mile = 1000 Rear Tire Diameter (Inches) = 26.53 Differential Gear Ratio = 3.31 Drive Gear Teeth Number = 15 Computation Results: Computed Driven Gear is 38 Ratio of Driven Over Drive Gear is 2.5 Bruce Quote Link to comment Share on other sites More sharing options...
stangeba Posted July 20, 2014 Share Posted July 20, 2014 Found this info: # of teeth Gear color GM part # Shaft size Clip size 8 Black 6261783 1.19" 5/16 10 Purple 6261785 1.19" 5/16 18 Green 6260037 1.19" 7/16 19 Natural 8640188 1.19" 7/16 15 Grey 8642620 1.19" 5/16 17 Red 8640517 1.19" 5/16 Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted July 20, 2014 Author Share Posted July 20, 2014 Sam, I was told the drive gear is 15 tooth on all stock TH400 transmissions by the Rep at TCI. My assumption is the drive gear is 15 teeth. The rear axle is a 331:1 as I had it installed when axle was rebuilt and adding the 43 tooth driven gear with taller tires put it very close to optimal. That being said, maybe I should attempt to try different calculations of drive gears with that 43 tooth driven gear, axle and tire size to see if any of the other drive gear sizes matches up. I just don't remember and did not think to document those adjustments at that time. I have also got the GPS fully charged and will compare the MPH from it to speedo reading today to verify my suspicion. Considering you came up with same tooth count using my specs, I'm beginning to believe the drive gear count is not the correct one. Appreciate your input as that info is focusing me closer to a solution. Will advise after I had confirmed GPS and speedo reading today. Doug Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted July 20, 2014 Author Share Posted July 20, 2014 Borrow a Garman and see how close the MPH is but I also came up with the same gear they did. Unless you are the original owner and know everything that was ever done to the car, you are just guessing. Verify the axle ratio and not just by checking the tag, rotate the tires both one turn and carefully count the drive shaft distance. Input Parameters Are the Following: Speedometer Revolutions/mile = 1000 Rear Tire Diameter (Inches) = 26.53 Differential Gear Ratio = 3.31 Drive Gear Teeth Number = 15 Computation Results: Computed Driven Gear is 38 Ratio of Driven Over Drive Gear is 2.5 Bruce Thanks Bruce, I am going to use the GPS today. Yesterday I tried to on my trip but battery was low and failed before I had the chance to compare and auxiliary power cord was left at home. If things can go wrong, they usually do! Preparation, Preparation, Preparation is the key! I suspect the drive gear number is not accurate and will try some comparisons using the old 43 tooth driven gear, axle ratio and previous tire size with available drive gears to see if any matches. Might determine the drive gear I have. We'll see once speedo verified against GPS. Thanks for the info. Doug Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted July 20, 2014 Share Posted July 20, 2014 Doug, did you notice what color your drive gear is when you changed the driven gear? Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted July 20, 2014 Author Share Posted July 20, 2014 No I didn't Sam. Didn't know or think of looking if I could see it when driven gear was removed. I'm going to start by comparing GPS to Speedometer in a few minutes. Once I can confirm those comparisons, will start from there. Will advise my findings when I return home this afternoon. Doug Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted July 21, 2014 Author Share Posted July 21, 2014 Okay guys, Drove the freeway today using GPS and comparing against the other. What I found was at 80 MPH speedo, GPS read 65 MPH. So, my fear was confirmed. Of course at slower speeds that variation lessens but guessing the percentage would remain constant? Also, based on calculation my Tach @ 65 MPH said it should read 2720 RPM when actually read 3050. I am going to assume some variation in accuracy of GPS vs. speedo along with my accuracy of reading these 3 parameters simultaneously? Using online calculators and comparing with my previous tire setup to determine drive gear in it, the closet matching result was with a 19 tooth drive gear and the 43 tooth driven gear that was then in it. So, need a 15 tooth drive gear to adjust current variation with the 38 tooth driven gear currently installed. Expect to have this done prior to WM when I can really critique its accuracy! I expect it will read quite close then! Doug Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted July 21, 2014 Share Posted July 21, 2014 Wallace racing calculators says.... for RPM given 65 MPH 3.31 ratio 26.53 tire.....2724.85 RPM's now if you go... 65 MPH 26.53 tire 3050 RPM's.....comes to a 3.70 ratio I do not agree with the last one. I have a 3.73 and a 27.5 tire and 65 mph is approx 3300 rpm but I do have a 3000 stall converter 3050 rpm's seems high to me for a 3.31 gear @ 65 mph what did your speedo read with the 43 gear? Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted July 21, 2014 Author Share Posted July 21, 2014 Yeah, like I alluded to previously. I am pretty sure my observation was skewed somewhat trying to watch speedo, GPS and Tach performance at the same time. I believe once I get the correct drive gear in, it will make it much easier to get an accurate reading or at minimum close to accurate. Doug Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted July 22, 2014 Author Share Posted July 22, 2014 Attempted to correct speedometer overage reading by exchanging drive gear. After some preliminary investigation, discovered work content would require more disassembly than I had planned for. There was available to me a external reduction adapter that I installed. The reducer was .89xxxxx and @65 MPH GPS, brought my speedometer reading down to 71/72 MPH. At 18 MPH GPS, speedometer was showing 20 MPH and at 35 MPH GPS, speedometer was reading 38 MPH. I am comfortable with these numbers for now. Now I will test odometer reading for accuracy. Until really looking underneath car, did not anticipate rear cross member / exhaust interference. Could have been done but was not prepared to devote that energy or time and the reducer adapter was quite easy to install. I have to figure out mathematically what reduction adapter will dial me in. I might just remove current adapter, hook back up speedometer cable to tranny and I think based on that math I would need a 17% plus reduction adapter. The one available was handy and used it to see what change was made to speed. The result was expected and reduced speedometer reading closer to that of GPS reading and within 6/7 miles vs. the 15 mile variance before reducer. Not sure when I will tackle this. Again, am comfortable with current discrepancy but at some point will try to dial speedo reading in for accuracy. Thanks to all for your advise and suggestions. I will advise once I have gotten this dialed in to my complete satisfaction. All have a GREAT day........... Doug Quote Link to comment Share on other sites More sharing options...
Guest RichardKn Posted February 8, 2015 Share Posted February 8, 2015 I have a 94 civic si coupe, and about 6 months after I bought it the speedometer and odometer started to only work about half the time. It seems to work when it is warm out or once it gets up to temperature. I had thought it was the speed sensor so I replaced it and nothing changed. Im fairly stumped, Ive been told that I may need to replace the cluster but it has such low ks for a 94 I doubt if ill be able to find one around the same number. And I dont want to change it unless I know thats the problem, does anyone know what my problem could be? Any help would be greatly appreciated Quote Link to comment Share on other sites More sharing options...
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