March 18 2010
Rear End Gear Ratio RPM Calculator
Tagged Under : 700r4, calculator, cruising speed, Eaton, gear ratio, HO72, rear end, rpm, tire size
This Suburban is a 3/4 ton version, so it's very heavy duty, made for pulling things, off-roading, and basically not driving at constant 2010 highway speeds (it is 40 years old). With a Turbo-Hydromatic 350, three speed transmission with no overdrive, and the 3/4 rear end gears, it really spins at high RPMs on the highway. The rearend in this '71 is an HO72 full-floater from Eaton, and I believe they only came in 4.11 and 4.56 gears.
Long story short, even with the 35" tires on it, highway speeds really spin the motor out. Tires that tall are a bit big for my tastes anyway, so if I go with a 33" or 31" tires the motor is going to spin even faster at any given speed. I've got a couple of options... numerically lower rear end gears, or an overdrive transmission like the 700R4, or both.
To help me figure out which route I want to go, I made this little calculator. Input the cruising speed, and you can calculate the RPMs at six different GM truck rear end gear ratios, with three different tire sizes, and two different transmissions.
Cruising Speed: mph| Tire Size: | 35" | 33" | 31" | |||
|---|---|---|---|---|---|---|
| Transmission: | TH350 | 700R4 | TH350 | 700R4 | TH350 | 700R4 |
| 4.56 | ||||||
| 4.11 | ||||||
| 3.73 | ||||||
| 3.42 | ||||||
| 3.08 | ||||||
| 2.73 | ||||||
I have RPM values that fall between 2,000 RPM and 2,500 RPM hilighting in green as I figure that's the optimal cruising range for a 350 in a giant brick-like vehicle such as an old Suburban or Chevy truck.