1978 full-time 4x4. Is a 700r4 worth it?

[Jankovic]

Originally Posted by D13

Do the math.
TH400 to 700R4, with lock up, is about .7 ratio change (since the lockup eliminates the slip of the converter too).
Lets say getting 10mpg. 10/.7=14mpg.
Let's assume its' your DD and sees 15,000 miles per year. 15,000/10 = 1,5000 gallons of gas, at $3.50, that $5250 a year.
15,000/14 = 1070 gallonss, at $3.50, that $3750 a year in gas.
So that's $1500 a year, assuming you can drive it at 60mph and 1540! RPM. Which you can't - you need to be around 1650 rpm at 60mph to tolerate the drive. Really, you need to go to a part time case to get reasonable fuel economy.
Here's the key. Aim to make 1600RPM at 55MPH, and find the ratios that work. Think of the 700R4 as a .72 ratio and play with tire size and gearing.

Too much 'Murican math there! I need KM's and Liters! Just kidding haha. Thanks for the feedback.

I inputted those numbers into a RPM/Gear ratio/Transmission calculator and it said for 1600RPM @ 55MPH I would need 4.10 rear end gears with a 700r4.

Wouldn't that "lug" the motor? I mean, this is a typical speed I would drive to work so it is ideal to make that speed the most efficient.

If I put 4.56 gears in, it says that I will run 1750RPMs @ 57MPH. That seems better. What do you think?