I will be doing a 700R4 swap. And went to 2 sites that have speedo drive and driven gear calculators. BTO's and the one on the PATC transmission site. I have 275/40/17 rear tires and running 3.55 gears. When I measure the height of my tires I get a height of 25". BTO says that I need a 15 drive/41 driven set. PATC says I need 15 drive/43 driven set. Why the difference and which is correct? :confused:

Your tires are rated at 25.7" tall, my guess is one site used your measurment and the other used the tire's rated size. If I recall there are spreads in rear gearing also, I switched from 3.73's to 3.42's with my 26" tall 255/45's and the same drive/driven combo came up for both rear gears.

Your tires are rated at 25.7" tall, my guess is one site used your measurment and the other used the tire's rated size. If I recall there are spreads in rear gearing also, I switched from 3.73's to 3.42's with my 26" tall 255/45's and the same drive/driven combo came up for both rear gears.

I went back to both sites and entered in just the rated size that you gave me. BTO was still says to use a 15/41 tooth gears, PATC numbers changed. But says to use 15/42 tooth gears. How much off would I be either way. Presently with my TH350 w/3.55 rear gears and the tire size listed above my speedo is around 25-30 mph off (to fast). When I checked against the mile markers in one mile my odometer registered about 1.4 miles. My old rear end gears were 2.73's.

See what this one shows:
http://www.chevelles.com/shop/speedo.html

I went through the same dilemma.

The gears are very close and the choice is whether you want it 2-3 mph too fast or 2-3 mph too slow. I picked too fast just as the factory sometimes did when they picked the gears.

See what this one shows:
http://www.chevelles.com/shop/speedo.html

I did the calcs. and my numbers came up to be 2.790***. Which gave me a 14/something I don't remember. But the lowest tooth for the 700R4 is 15 drive gear. Which bumped it up to 2.8 and the gear selection would be 15/42 by that calc.

I went through the same thing. Mine was close to begin with but I eventually ended up swapping the driven gear twice to get it perfect.

The driven gear is the easiest to change and only costs $8.

According to chart mine calls for 13/38 or 12/35 big gear? I don't know if I have big or small. It's a 69 w/late model 700r4. Didn't know you could change speedo gear & would rather not since it's new. I have 370 gears w/25.68 tire height. Any suggestions?

Scroll down this page for gear availability
http://chevelleengineer.home.comcast.net/~chevelleengineer/speedo.htm

Something to keep in mind ,GM didn't make speedo gear sets for tire sizes not availably on their cars.
If your's is an odd ,non-GM tire diameter you need to settle on a gear set that's "close enough" or get a ratio adapter as shown here.
http://www.transmissioncenter.net/speedometer_calibration_______va.htm#gm%20ratio%20 adapter
Just like in the old days ,when you change to non-stock stuff ,you need to re-engineer it yourself.