olds 307 and 403

Ok, so I am installing a TH350C in my 70S. I have 2 types of housings and 3 different driven gears along with 2 different drive gears. Going off the TCI calculator, my original speedo gears were way off, assuming I am counting right and their colour to teeth chart is also correct. The original trans had a green drive gear, still in cars and big blue driven gear. Of course there are two blue gears, 20 tooth and on 36 tooth. By the size, it has to be 36 tooth, which is way wrong for 215/75R14 tires, which close to stock height. My speedometer seemed fairly accurate, maybe it was 10 mph or 10 on off, can't remember with the TH350. Either way, I have two natural 19 tooth driven gears, double checked the tooth count on the one I am using, it is 19. I plan on using my TH250C tail shaft, fairly new bushing and seal. The early and late housings have different speedo bullets. The 250C and 350C have the same size as the 2004R, the earlier one is much larger, maybe it changed the driven gear position? My question is, can I use the small natural 19 tooth driven gear with the 9 tooth green drive gear? Also do I count the part teeth or just the full teeth on the drive gear? I get 11 teeth on that green drive gear, which I assume is the difference. I want my speedo accurate, that combo should be very close.

4speed455

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

Looks like your on the right track. I assumed your rear gears based of your signature. With those tires and gears the calculator says to use the green 9 tooth drive gear with the natural 19 tooth driven gear. So you should be good.

olds 307 and 403

Yes, 2.78 gears and I got the same numbers. I counted my green gear and counted a purple gear as well. Counting them the same way, not counting the partial final tooth on each side, I got the correct 9 and 10 teeth.

olds 307 and 403

Well, somehow my speedometer is way wrong with the 19 tooth natural gear and the green 10 tooth gear. I am talking 70 mph is 50 km/hr! I have blue gear with 38 teeth, not 36 teeth driven gear and the green, blue and purple drive gears. I can't even get on the TCI site right now to figure this out.

oddball

Re: speedo driven housings: the large one is a BOP thing (mostly) on the TH350. Chevy used the small bullet housing on the TH350. Only small driven gears fit through those small openings, so those setups tended to use large drive and small driven gears. Humorously, the 700-4R switched back to the large housing, so sometimes just look for 700-4r gears.
classictransmissionsolutions.com has an impressive selection of parts

olds 307 and 403

I am going back to the stock housing with the 38 tooth driven gear and green 9 tooth drive gear. The calculators show it should be no where near right, I believe it was 5 or 10 mph slow, at least it is close. I also have a 7 and 10 tooth drive gear, I plan on buying the 8 tooth drive gear as well, so I can fine tune. It is the later trans that have the bullet gear. Both my BOP pattern TH250C and the current dual pattern TH350C have the small speedo bullet extension housing.

RetroRanger

The speedo will be off a percentage, I swapped my 2:7x rear gears for 3:42s, my speedo is off about 23% now w speedo reading fast.

20% is easy to calculate but 23is a pita. Until I get time to fix this I made a chart and printed it out, I put it where I can see it easily while driving.

30 mph reads 37
55 mph reads 67
70 mph reads 86

Before swapping the rear end the speedo was accurate according to those roadside police signs that display your speed

olds 307 and 403

Mine was maybe 10 mph slow according to the GPS, going by memory. I will fine tune with different drive gears, using a calculator was no help. I also need to check for 12 volt at the lock up plug, no lock working. I run 2350 RPM at 60 mph, locked up, it should run 2050 to 2100 RPM at 60 mph.

olds 307 and 403

Well, my speedometer is 5 mph slow. That is the 38 tooth driven gear and a 9 tooth drive gear. I hope a 8 tooth or 7 tooth drive gear will put it bang on. Also the Super Tach reads 150 RPM higher than my timing light at 60 mph.