My 78 F150 4x4 with a brand new 400 runs about 2700 rpm at 70 mph. It has the C6/205, 9"/D44 and 35x12.50x15 tires behind the 400, can anyone give me an educated guess at what gears are in it? It was 3.50 stock and i was assuming it was still stock but 2700 at 70mph seems kind of high, if i went with a shorter gear it would really be screaming at 70mph.

Also if indeed i have the stock 3.50 gears what is a good ratio to use for my setup, and keep it drivable on the interstate on occasions. Also where is a good place to order gear sets for the 9"/D44. Thanks

 

It is most likely still at 3.50, I don't know what the final ratio's are with the c6 but I have a np435 np205t-case and 4.10's and at 70mph I'm at 3,000 with 33's... you can jack up the rear and spin the driveshaft, count how many times it goes around vs. one rotation of the wheel and you can get a rough estimate of what ratio you have

 

OK thanks for the info,your 4 speed must have more of a hiway gear than the c6. My truck still has plenty of pep and will push you back in your seat but i think re-gearing might really help out, especially in deep mud, which i plan on doing some mudding this spring/summer. Would a 4.10 or 4.56 put more stress on the axles in mud or less stress? I was thinking about that and couldnt really make up my mind. I want to keep the 9"/D44 for now and I really dont want to stress the axles any more than they already are, with 35" tires and 350hp.

Is there a vendor that sells a kit to regear these trucks or what all do i have to order? is it a huge deal to install a limited slip while i am in there? It is an open diff right now, usually just one black mark lol.

 

Well I have a d60 out back but for the most part I think its pretty much the same...just smaller. You need a new ring and pinion to change over, and there are bearings in there that will need replaced. If I were you I wouldn't go any larger than 4.10. I put a detroit locker in mine, its amazing and I prefer this over limited slip. I know sometimes limited slips don't engage so well. So maybe do some research into that. If you do it all at the same time it will make things really easy for you. Frankly i worry about my d60 and breaking axles but I think your automatic transmission will benefit you there, the torque converter is a much more beautiful thing then a clutch, not as rough on the rest of the drivetrain. Back to the rear end, I purchased my stuff from 4 wheel performance because they are in town but it should be absolutely no problem to find a 4.10 kit. Remember if you do that you need to re-gear your front diff too otherwise when you put it in 4 wheel drive you gonna be a hurtin unit. A 4 wheel drive shop should be able to tell you what ratio you need, most trucks have a slightly smaller ratio up front like 4.10front 4.11 back (something along those lines).