What axle ratio would you want with the 4BT/TH400 to give a decent cruise?
Best to figure your own gear ratios and speeds by plugging in all the variables, tire diameter included. I like this gear calculator:
http://www.onlineconversion.com/bigger_tires.htm use the last on on the page.
The 4BT is supposed to get good mileage at about 1,600-1,700 rpm, assuming the truck is not really heavy. if really heavy the RPM at cruising speed will need to be higher.
So, some rough figuring: 33" tires, no OD, 70 mph, and 1,650 rpm says you need a 2.3 rear axle ratio.
Best find an OD transmission, or really big tires... play with the variables till you find a combination that gives you want you want with suitable and available parts.
The idea is to be at cruising speed on the freeway with the engine at the RPM, or just faster, that it makes it's peak torque. Then it gets good fuel economy and is very drivable. Either under or over geared waste fuel, and don't forget about the weight of the truck and the HP available, they have to be compatible too, (a single cylinder lawn mower engine won't move a pickup down the highway at 70 mph).
Grigg
