30x9.5R15 is about as far as the 3.55 will handle, 3.73 would be better. Anything larger should really regear to 4.10 or lower and will likely require lifting and/or trimming to avoid rubbing the front wheel wells on turns or bumps.
The 30" LT tires are about twice as heavy as the passenger 28's so braking will be affected. At lower speeds (like under 40) you wont notice too much but highway will definitely take longer to stop. The truck will loose some acceleration and might feel somewhat underpowered, but you should feel much more stable with the heavier tires, wider footprint, and more aggressive tread pattern. Speedometer will read 7+% low so if it says 60 you are actually driving 65 unless you change the revs per mile setting in the computer.
For a few months, I ran 31x10.50's on factory wheels with 3.27 gears with my 94 5-speed. It was no speed demon, needed more distance to stop, but it did it just fine. For the last month, I have had 31x10.50's on aftermarket wheels on my 92 with 3.27's and an automatic. I NEVER use overdrive, and try to take it easy on it. It does well enough, but like I said, I only use drive, and try to be as easy on it as I can. For a few years I ran 31x10.50x15's on my 94 5-speed with 3.73 gears. It drove just as it did with the dinky 235/75/15's and 3.27 gears. Same mileage, speedometer was correct, same overall vehicle performance. I have also run 33x12.50x15's multiple times on my 94 with the same 3.73 axle gears. On the road and on the trails. $.10's would be better, but I can do anything I want. Use 4-lo off road and I am just dandy for the little bit of trail riding it gets to see. On the road, once I am moving, I have plenty of acceleration. Starting off kinda sucks, as it did when i ran the 31's with stock gears, but it makes it just fine. And I beat on it like I'm trying to blow it up. 31's on both vehicle required no lift or trimming. 33's only required a 2 inch lift with small amounts of front fender trimming and liberal liner trimming.
My point to all this is, there is NO set ratio that you HAVE to have in your rig to run a certain size tire. You can run 40's with 2.73:1 gears, if you have the engine to back it up. I ran 31x10.50x15's on the back of my 78 2wd F150 for a long time and it had 3.00 gears (what we were told they where switched to). That truck did have a big block 460 in it, but I could do a burn out (read: one wheel peel) like it was nothing.
TO THE ORIGINAL POSTER: Bottom line is we all learn by some trial and error. We all also have our own preferences. Try what you want, if it doesn't feel right or work the way you want, lesson learned. Hope this mini-lecture is helpful in some way. Happy motoring!
I'll post back here to let everyone know what Ford says lol.
Thanks again for all the advice.
