Quote:
Originally Posted by BK'66Step
So I just cracked open my rear diff for the first time in preparation for cleaning it up. I discovered that it has 3.08 gears!  So my end result is to have a SBC 350 with 350-400hp, 700R4 trans, and 255/60R15 tires in the rear. I was wanting the 3.73 gearing but, if I understand correctly, the lowest I can get are 3.42 gears in a 3 carrier rear end. Does anyone have a similar setup with 3.42 gears that can comment on the drivability or suggest another setup. Any other thoughts would be appreciated as well. Thanks all! |
I use this web site to calculate RPMs at different speeds.
LINK Go to the bottom section and enter the rear diff ratio (3.73 or 3.42 or whatever you have), then the tire height, the MPH (I used 70 mph), and your final tranny gear (which is 1.00 for the 3 speed factory tranny). If your tranny has OD, then you will need to figure that out. A TH700R4 final gear is 0.70)
What engine you have is not a factor except for what torque it can produce at certain RPMs and the strength of the tranny.
In the example above, a TH700R4 with a 0.70 OD gear and a 3.73 rear end and 29" tires will have your engine doing 2118 RPMs at 70MPH. That's a great number. If you went to a 3.42 rear diff (That's what my 66 parts truck had), then the engine drops down to 1942 RPMs.
I think a 3.42 rear would be about perfect for you.
I have a 0.63 OD on my manual tranny, so a 3.42 rear end will not work well for my setup.
If I had stayed with my factory 3 speed which has a final gear of 1.00, then the 3.42 would be a tempting swap. It would give me lower RPMs at highway speeds but at the sacrifice of less power in 1st gear.
You can calculate the rear diff ratio without removing the rear cover.
1. Chock the front tires and put the tranny in neutral.
2. Jack up the rear axle so both rear wheels are off the ground.
3. Mark the DS tire at the 12 o'clock position.
4. Mark the drive shaft so you can count revolutions.
5. Slowly rotate the driver's side tire and count the total tire revolutions while a helper counts exactly 20 driveshaft revolutions. (or any multiple of 10 - the more the better).
6. You should see that the driver's side tire spins and the PS stays still.
7. The tire will spin twice as many times as it normally would since both tires are off the ground.
8. The math is easy. Divide the number of shaft turns by the number of tire revolutions and then multiply by 2.
Example: 20 shaft revolutions divided by 11.75 tire revolutions = 1.70213 times 2 = 3.404. Those were the numbers I just got 2 weeks ago when I calculated mine. So it means the rear was a 3.42 rear.
Another way is to roll the truck in neutral and count the revolutions (but you don't multiply by 2 at the end). This method is kinda tough because it's harder to count shaft revolutions on a moving truck.
Anyway, I hope all this makes sense and is helpful.