I'm using a 1997 F250 heavy duty. 460 7.5 5spd manual. I have the 4.10 rear end. Extended cab. 72k miles
I tow a pretty small Terry 5er 22'. Depending on what is in it it weighs around 5500. On DOT scales. Fully loaded I'm 900 lbs under max payload. Max trailer weight for the truck is right around 10k. I've done close to 8k and it wasn't bad even in the Wyoming mountains.
I also tow a gooseneck horse trailer with 3-4 horses all the time with it.
It is paid for and what I could afford.
Ford does not recommend towing in 5th/OD. So towing in 4th mainly keeps the speed down unless you want to run your rpms up. Although I will shift into OD going downhill at times unless I need 4th to keep my speed down. I generally cruise along the highways at 55-60 while towing. So I won't win any races and it will slow down some on the hills. Pretty reliable motor and transmission.
I get around 13.5 mpg empty going 65 and 9.5 with the fifth wheel going 55-60. I think that is above average for this motor, but I live 30 miles from town, no traffic around me and I drive pretty conservatively.
We like the truck, wish it was a crew cab. No frills just a truck.
