Well, I have a stick shift, but I'll say this - 0.75:1 accomplishes little improvement and is a waste of time and money to convert. What is worthwhile is 0.5:1.
Here's what I mean. Top photo is direct drive 1:1 at 60 mph, 2,750 rpm
Next photo is 0.75:1 at 60 mph, 2,300 rpm, a savings of 450 rpm. Meh.
Bottom photo is 0.5:1 at 60 mph, 1,550 rpm, a savings of 1,200 rpm. Perfect.
This is what you get in the cockpit in real life, not what the math says it ought to be.
Sometimes when the RPM is too low, it lugs the engine and actually gets worse mileage than slightly higher RPM. The O/P has a 318 truck and may not like too much OD reduction.
