estimating gear ratio

[61_FL_Apache]

I was trying to figure out my gear ratio by counting the revs between the driveshaft and tire. was that how you do it?

if thats the case, I got one rev on the tire to two revs on the driveshaft. That doesnt seem right.

[jocko]

Am guessing you do not have a posi (to confirm you do NOT have one, jack up both rear tires off the ground, turn one one way, if the other turns the other way, you do not have a posi. If non-posi, and it was just over 2 revs of driveshaft, then probably a 4.11, if just under 2 revs of driveshaft, then probably a 3.73 - but here's the procedure:

from http://www.how-to-build-hotrods.com/gear-ratio.html

Posi: Jack both wheels off the ground. Turn one wheel 1 complete turn.

- Count the turns on the drive shaft.

Non-posi: Jack one wheel off the ground. Turn the wheel 2 complete turns.

- Count the turns on the drive shaft.

If it takes about 2 and three-quarters turns, it's around a 2.73 gear ratio. If it's a little more than 3, you have 3.08s. If it takes about 3 and a quarter turns of the driveshaft, you have 3.23 or 3.25 gears. Very close to 3 and three-quarter turns, 3.70 or 3.73 gears. This method won't work very well on a posi that's worn-out and won't spin both tires equally.

[61_FL_Apache]

I dont have posi. I had 3 1/4 turns on the shaft to 2 turns on the tire.

What does that give me?

I know that the rearend has been changed and its a 12 bolt. I dont know what this rear is out of.

[jocko]

then that would probably be a 3.23, which is not a stock (for 60-66 chev trucks) rearend ratio. Is your rear end orignal to the truck or has a swap occurred at some point. 3.23 is not a bad gear ratio... Also, make sure the "other" side tire is on the ground or chocked somehow to get a true reading. But if you got 3 and 1/4, then you likely have a 3.23. That's not a bad ratio to have, by the way - decent mileage/perf compromise. Stock for a 60-66 was 3.73 with optional 3.08 (and 4.11 I THINK, but don't quote me on that).

Depending on your trans 1st gear ratio and whether or not you have an o/d trans mod in the truck - 3.23 is pretty good. If you want to fry your tires, won't work. If yiou want GREAT mileage, won't work - but a good ratio I think anyway. (again, it all depends on your trans and its ratios and what you want to do).


OOPS - MY BAD - I SEE IT IS A 12 BOLT and NOT ORIGINAL. THEN YES, probably a 3.23.

[61_FL_Apache]

Thanks Jocko for the info!! The mileage and performance is good. With the mild 305 and TH350, I have no complaints, just curious. I would like to go to an 700R4 the next go around.

At 65 I turn 2500rpm with a 215/70/15 tire. I would like to go to a taller next time also and that will bring my rpm's down too.

[fastwillie 696969]

open diff with probly 4.10 rear gears just sayin'

[61_FL_Apache]

Its not that low.

[jocko]

willie - how do you come to that ratio? Just curious?

[61_FL_Apache]

Ok!! I finally got into my rear diff today. Its a 3.08. A buddy of mine had a spare axle and we changed it out today cuz I had a bent one. The stamp on the ring gear also has a 2/69 on it. I take it thats the manufacture date. I know it was at least a 65 to something.