How to determine transmission size and gear ratio on Ford TV

My '02 F250 with 7.3L & 3.73 rear end & auto tranny handles our 10,500 lb 5er no problem and I would not hesitate to pull a 12,000 lb 5er. :)

Startability up a steep grade is the concern with rear ratio. Around Alabama and the Blue Ridge Hills, you might see an occasional strain to get moving. Other than that, I look at GVWR, GAWR and tire ratings. I like a gear ratio that puts the engine in it's power band on the flats and up the hills at 55-65mph. For me that's been the 3.73 both gas and diesel. Diesel in O/D and direct. Gas in direct and it's first under drive ratio.
My '01.5 250hp Cummins Ram had a 3.55 rear ratio. No problem towing our previous 10,400 lb weighed CAT scale weight, 11,400 lb GVWR high profile 5th wheel. It would downshift on the uphills to run 2600rpm but that wasn't a problem for me. It ran 1750~rpm in O/D. It was rated to tow 9700 lbs.
With the Ford and stock tires, if you're running 1750~rpm at 65mph, probably 3.55. If 2000~ rpm at 65, likely a 3.73. If 2200~rpm at 65, likely a 4.10.
MPH X rear ratio X trans ratio X 336/tire diameter(30.5~) = rpm.

OP, I have a 02 F250 PSD 4X4 with the manual tranny and 3.73... great truck. Mine has the camper package that adds the rear F350 helper spring and sway bar on the rear. Check and see if you have these two items as then you should look at the F350 ratings. Unfortunately this does not change the door sticker as I sometimes gross over 26K pulling an equipment trailer where I can adjust the pin weight. You could add the extra 350 spring as the axle and brakes are the same as a F350.

I pulled a 34' Carrie Lite fifth wheel from Florida to Alaska back in 1996 with our new F-250 7.3L PSD. No problems. In fact, we had over 200,000 on that single wheel F-250 when we traded it for our present truck. We have owned two F-350 PSD and both of them were and are used exclusively for towing our fifth wheel. Our present 2000 Cameo has over 90,000 miles on it rolling along behind our F-250's.

Maximum trailer rating is a warranty issue. Not likely you still have any warranty to be concerned about. Just watch GVWR and GAWR as those are safety related issues.

Try any ford VIN decoder to see what the spec are on truck. And also look at door sticker of "axle group" Then there should be a axle tag on rear diff. get the numbers off it and look it up.

http://www.google.com/#hl=en&gs_rn=12&gs_ri=psy-ab&cp=7&gs_id=s&xhr=t&q=ford+vin+decoder&es_nrs=true&pf=p&output=search&sclient=psy-ab&oq=ford+vi&gs_l=&pbx=1&bav=on.2,or.r_qf.&bvm=bv.45960087,d.cGE&fp=771fa58581d29b28&biw=1366&bih=622

http://www.google.com/#hl=en&gs_rn=12&gs_ri=psy-ab&pq=ford%20vin%20decoder&cp=13&gs_id=3p&xhr=t&q=ford+axle+tag+codes&es_nrs=true&pf=p&sclient=psy-ab&oq=ford+axle+tag&gs_l=&pbx=1&bav=on.2,or.r_qf.&bvm=bv.45960087,d.cGE&fp=771fa58581d29b28&biw=1366&bih=622

Takes awhile but worth it. I thought I had a 4:10 ratio but using axle tag have 3:73 Also while under there changed diff grease/oil. Never thought I would spend $20.00 a quart for oil but that is what it goes for.
Hope this helps.

Forgot to mention that the truck is in excellent condition and only 75000 miles on it. I really don't think that is too old.