Not real clear on how to determine what speedo gears I need for correct read - I'm running 3.23 posi with 235-60-15 front and 255-60-15 rear tires - currently my speedo reads approximately 18% high (for example, speedo reads 60 at roughly 50)... Car originally came with 2.56 gears are there any charts out there that outline gears needed for particular setups (possibly with part#s)? At worst, I'd at least like to get closer than 18% light.
You are doing the same speedo gear swap that I did a while back - I also added a 3.23 posi with 255x60x15 tires. If you do a search on the board you should find my old posts somewhere. I think that I used a purple 43 tooth driven gear (#1362196) and a new sleeve which accepts 40-44 tooth gears. I had a green 18 tooth drive gear and a pink 35 tooth driven gear. You need to check your tooth drive gear to see if you need the same. Once I did the swap (which was very easy - by the way, I have a 350 tranny - if you have a 400 tranny it will be a different story for your swap) my speedo read correctly.
thanks for the great info - I'll look further into it - I've been agonizing over whether to go for a 2004r swap but $$ dicates otherwise right now - and since I just snagged a newly rebuilt '70 vintage BOP TH350 for well below build price, this is the best time to get it all right. :beer
If you have any questions on the speedo gear swap ask Larry AKA "The Wizard" - he helped me a lot with this process.
Front tire size doesn't matter. What you need to know is rear tire size, rear gear ratio, and the number of teeth on the speedometer DRIVE gear. With those 3 things, it is easy to figure out which DRIVEN gear you need. Pull the DRIVEN gear sleeve out of the side of the trans. With the rear of the car in the air, trans in neutral, look into the sleeve hole, and rotate the drive shaft by hand. Count the number of teeth on the DRIVE gear. Do it several times to avoid making a mistake.
TH350 could have either of 2 types of speedo gears, depending on the tail housing. One has driven gear about 7/8" in dia. The other tail housing uses TH400 type driven gears, about 1 3/4" dia. These have about twice as many teeth, so its easier to get the speedo just right. Either setup can be swapped on without removing the trans. For a 2.56:1 axle, I would expect the speedo gears should be close to a 2.0:1 ratio, to get 1001 speedo cable turns per mile. That could be around 19:38 for the larger dia gears; perhaps 10:20 for the smaller gears. To convert to a 3.23 axle, with a 19 tooth drive gear, I'd expect to need a 45 tooth driven gear. Check your old ratio & make corrections. Bruce Roe
Well, I know I have the short tail and the TH350 I'm swapping in from the '70 SP drivetrain is identical. Would presume it has the same gears as in my original unit so will definitely check it out. Thanks!
I've seen a chart for both teh TH350 and TH400 that gives the part number and description of all the drive gears, driven gears and adapter housings. I'm sure out there on teh internet somewhere. There is a link somewhere on the board that I could swear I've seen before. And you are sure about your speedo error? The math validates the numbers?
Ugh. I think this is why my speedo died after I installed a new TH-350. I did not know about this little detail.... I have a yellow driven gear (removed from the old tranny)- so which drive gear should I have? I am sure that when I open it up tonight, the yellow driven gear is going to be chewed to pieces.... How difficult is it to replace the drive gear- do I need a new tail housing? BIG question: Can you physically install the wrong size driven gear in there- I mean, will it fit and then fail in service, or will it simply not slide in at all? Thanks for any info.
Ok so here's an update - I pulled the speedo gear housing off the TH-350 and it's a light green 45-tooth gear - housing is p/n 1362236, marked for 40 to 43 teeth. The sleeve inside the housing is dark green and I could only feel 8 grooves top to bottom in the limited access I had. The TH-350 is reportedly the matched trans to my 70 SP 350 motor, so I'm trying now to figure out if it's any better matched to my 3.23 rear than whatever is in the '72 transmission which is still in the car. I'm currently about 18% slower in actual speed than the speedometer readings (at least up to about 70mph). Any thoughts?
That 45 tooth driven is as high as it goes. So you'll need to reduce the number of teeth on the drive gear. Not sure what green is, but turn the drive shaft around & count the teeth going by a few times. Divide that by 1.18 to determine what you need. If you overshot a bit, the driven gear can be reduced a tooth or 2 to fine tune it. Bruce Roe
The Green drive gear should be a 9 tooth gear. With 3.23, and 27.1" tires, I get about a 22 tooth driven gear, but that is one of the small Chevy Gears. You will need to change your drive gear I think. Have a look here: http://www.tciauto.com/Products/TechInfo/speedo_gears.asp
heres another speedometer gear site. Calculate exactly what you need. http://www.transmissioncenter.net/speedometer_calibration_______va.htm edit: I just looked at larrys link, it calculates automatically....I like it better than manual calculation.
http://chevelleengineer.home.comcast.net/~chevelleengineer/speedo.htm In a th350 bop it looks like the green drive gear is a 17 tooth Looks like you will want a 42 green driven gear.
Thanks for the updated feedback - 42 may be about right because speedo is now only slightly off with its 45-tooth gear, where with prior trans it varied significantly. I never did yet pull the other TH350 gear out to see what it is but it was reportedly original to the car from when it had 2.56 gears - so I'm sure a different gear.
