I have been thinking about putting a transmission a little bigger tahn my 3 speed auto in my 1993 F250. I do a lot of pulling of very large heavy trailers and my transmission will do it but it often strugles with take offs with a big load. I want something that will be geared a little lower in 1st for take offs and a little higher for highway speeds. What transmission is going fit and be worth the work, or would it just be better to stick with what iv'e got.

 

Can't give you the gear ratios between your 3-spd auto and others, just what i've experienced with auto and manual: My 351 has an auto trans, and it did just fine except when pulling my boat (6,000lbs dry) up out of a very long, steep grade from a lake. A hot day, a neverending long hard pull, and the transmission overheated to the point i had to stop and let it cool, even with not using the A/C to avoid more heat to the cooling system. It also has an auxiliary cooling radiator for the transmission. The same with my 490, on very long hard steep pulls i can feel it getting weary of the strain, and it also has an auxiliary cooling radiator, which on occasion (last year on I-90) added enough heat to the cooling system that i had to turn off the A/C. This is with a load of about 18,000 lbs. On the other hand, the little I-6 4.9 has a manual transmission, and seems to do well loaded, not as big a load as the 351 or the 460, but then it's a smaller engine than they are. At any rate, the manual seems to not get as hot as the autos. But then, when you have no load, like out camping and chugging around the hills, the automatic does just fine. But for your slow takeof speed, it probably won't be much different in an auto as compared to a manual. And if you get stuck at a stoplight on an upgrade, with some fool nosed into your rear bumper, you'll be glad you have your automatic... ;-)

 

My big woory is that one day I will get stuck at that stoplight and not be able to take off at all. I pretty reguarly pull between 10 and 20 thousand pounds behind this truck and it is fine most of the time but I have beeen in about three uphill heavy load situations were I didn't think that the truck was going to be able to pull it from a dead stop. I'll get on the gas and will have to hit it pretty hard to get it to move at all and then it will take forever to get enough speed that I know it is going to make it. You can see pictures of my truck with a load in my photo gallery.

 

Maybe an AOD transmission- my understanding is it's a C6 like you've got, but with overdrive. Then change your rear gears to give a better 1st. The C6 has a really shallow first gear of 2.46:1, I think, which limits you. 2nd will be about 1.46:1, and 3rd is 1:1. AOD gives you a .80:1 overdrive.

A granny-low manual, such as the classic T18/NP435 4-speed or a rare ZF-5 speed will give you a great low 1st gear that will haul any load away from a dead stop. ZF gives a nice overdrive too (.76:1). Some 1st gear values: T18: 5.11:1, NP435: 6.69:1, ZF: 5.72:1. All are really strong transmissions.

Your easiest option, I think, to accomplish both your goals, would be to swap in an AOD and change your rear gears. It's still not electronic (like an E4OD), so it won't be really complex. A manual conversion will be a pain, requiring the clutch pedal assembly and cutting sheetmetal.

You may be able to get different gearsets for your C6 that would give you a wider gear spacing.