Hey Juspar!
Ok, I own a m109a3 and a m35a3. Your tire question is pretty simple to answer.
You are going to be changing the rims if you want to put bigger tires on. The A3 wheels are wider than just about anything (11 inches) so the majority of the tires aren't going to like that without a beadlock. The beadlock in the a3 wheels seems to be too wide for the beads on anything other than the 14.5x20's, so you're going to end up with either a new wheel or fabricating a bead lock. Or you're buying hemtt wheels and re-centering them. I'm still sitting on FMTV wheels since finding centers for that still ain't happening! One interesting note here, my tires are rated at 8000 lbs, but the A3 wheels I have say they are rated for 4000 lbs. Again, hemtt
Now, if you build beadlocks you can easily fit 46" tires on the stock rims and poof! you can now go 57mph.
BTW, my m35a3 will do 54, downhill. 52 in every other case, and loves hills. you CAN turn it up, but I'm not going to. One of the local CAT guys gets 300+ hp out of them, but the life expectancy drops to half or less. back to tires.
The 53's probably won't fit on your rear axles, even though it's **** close. Here's a good site to compare tire sizes, rim widths and stuff.
Army Military Tires
If you look at that, you can see what each tire relates to in inch size, and you MIGHT get 49's on (14.00 x 20). Someone can blitzkrieg me on that to let me know how wrong I am.
Lastly, check out my m309 trailer build. You might find you can make a camper out of a trailer pretty easy. All I needed was some metal, Nuts and bolts, 4 inch grinder and a winch.
Did I miss anything? let me know.
Jerry