Lest we confuse this with the very useful "crawl ratio" doctrine for anticipating low speed manners in traffic with standard transmission:
"The 1st gear ratio times the axle ratio should be above 10 for best results".
Example: 2.20:1 close-ratio Muncie with 3.08 axle = misery at 6.8, the 2.52:1 box is preferred here. The 2.95:1 T5 box works well with anything above 3.23.
How does this compare with the auto trans numbers? They all look too low? The missing factor:
1. stall speed means the engine never lugs.
2. the converter's 2:1 (more or less) torque multiplication adds mechanical advantage during roll out.
I hadn't heard about the 10:1 combined crawl ratio. It seems to consistently be the rear ratio that I call the hi-perf which is 2 steps deeper from the factory standard ratio:
My stock manual setup yields a 9.55:1 combined ratio which is close to the 10:1. My own experience with the stock engine is that it connects well from the crosswalk, but it is already bogging and wants to shift by the other side of the intersection. Then it rides 2nd up to 35 and starts to complain again until I can shift at cruising speed. So maybe it is asking me for that 3.08 rear. Then again maybe it just wants more advance or carb, we'll see how it behaves when I change those. IntegratedJ78 thinks it wants the extra gear of a Sag 4spd to go to.
Yeah, I knew there was something different with the auto. That makes more sense as to why they never changed the ratios in the TH350 despite the rear axle gearing getting higher for economy in the '70s. The Saginaw 3spd and 4spd manuals kept getting deeper and deeper 1st gears and that was to overcome the change to more highway economy friendly rear gears.
The slush-box just didn't need the gearing change......but so what is the optimal ratio that TonyPA is looking for then if crawl ratio is not important to his TH350?
Afterthought: Is the crawl ratio recommendation based on car tire size then? Because the Camaros and Novas came standard with E78-14/FR78-14/P205-75r14 which measures at 26" diameter. I know the larger cars like the Monte Carlo used the P215-75r14 tires, but I don't know what the standard size for truck tires was then? Obviously much taller.
