Automatic to 4 speed Conversion in '65 Formula S

Keep in mind, I use the 6.25 +/-.25 second gear ratio, cuz I like it at 32.5+/- 2.5 mph with 25.5 tires, it gets me a cruiser gear of about 2600 rpm, but more importantly is the backshift into 3.09-first which is ~4180 If you don't have the cam for that, then you're gonna run out of gear and have to go back to second pdq. That's two shifts and slows you right down. With a 2.66 low gear, it drops in at 3600 which is less power, but 2.66 low will go to 45 mph versus 39 with the 3.09 low. (5000rpm and 3.23s)
It's all about the compromises.
I built my last engine to work with those Commando ratios, and my GVOD, and couldn't be happier.
But yaknow, everybody is different, and my favorites may not be the next guys favorites. So with that in mind here is the formula I use; straight out of my head;
mph= (rpm x tc x TC)/(1056 x R1 x R2)
where tc is the tire circumference; also called roll-out, or diameter x 3.1416
R1 is the rear-gear, and R2 is the trans gear, R3 would be for a third reducer like the GVOD.
TC is correction for the convertor slip and is; 1/TM , where TM (Torque Multiplication) can be between 1.05 to as high as 2.0 I arbitrarily use 1.05 for small engines and 1.08 for street-SBMs. Except at zero mph I might use 1.5 to 1.8 depending on how much power I imagine the engine is gonna make, at stall.
Here is an example;
Mph = (5000rpm x 85 x 1/1.08)/(1056x 2.45 x 3.55); solve for mph
393518/9184.56 =42.8 mph. If the TC only slips 5%b then mph will be 44mph

At zero mph with a 360, the TC might put out a TM of 1.8 so then the starter gear could be treated as 1 x 2.45x3.55x1.8= 15.66 times the engines torque at stallspeed. With a 2800stall and say 300 ftlbs, this would work out to 15.66 x 300=~4700 footpounds into the rear axles, and instant tire annihilation..
The same TC behind a slanty might only have a TM of 1.6, and 120 ftlbs at 2800 so then
120x2.45x3.55x1.6=1670 ftlbs into the rear axles. So that's about a chirp plus. But as soon as the tires slip, that 1.6TM will begin to ramp down to something like 1.2 in first gear. So if she spins, as the rpm goes up more ftlbs is going into the TC, but it is being multiplied less. So say at 3500 the slanty is up to 140 ftlbs, then;
140 x 2.45x3.55x1.2=1461, not enough to sustain the tirespin
But if it doesn't spin then the TC will ramp down more slowly as the Rs go up, and the car begins to move out. In any event this large TM ramps down fairly rapidly. I'd be guessing but I think by one or maybe two car-lengths out it it has decayed to the minimum for that gear. Every successive gear will have less TM in the TC because as the road speed goes up the turbine and impeller are approaching synchronization. At WOT,there will always be some slip between them, and so always some TM. Typically I use 5 to 10% as an average for a streeter. For me it's all guesswork, based on common sense, and as long as I compare apples to apples, nobody is the wiser,lol. Yeah it hurts to say that I don't know everything.
Happy computing
BTW, with a manual trans, there is no TM so the factory usually gave the manual cars the next bigger rear gear, with an across the board increase in TM of about 10%. The penalty of course was a higher cruising speed.
And of course, with the same rear gears, autos are usually or at least often, a lil quicker out of the gate.