Hi first post here, Spent lots of time on RV.net and various diesel forums.

I am trading in my 08 F350 diesel for a 10 F150 XLT Screw 4x4 6.5 bed, 5.4l 3.73LS, max tow chrome pkg. I am pulling a 09 Passport 290 BH 5800# full for camping plus 900# tongue wt ( I actually went to cat scale), I used to have a toy hauler that was 4K# more so no longer need the big truck. I am planning on changing the tires to BFG AT/KO's but was thinking of going smaller than stock for a little more power to the ground. I have been pulling with diesels for 12 years and always went bigger than stock on tires, but this time I don't want to loose any power.

What do you think, will I gain any towing advantage with tires that are slightly smaller? Or will it just look bad?

LT275/65-18 BFG AT/KO's are listed at 32.09 tall 11"sec 9.2" tread
LT265/65-18 BFG AT/KO's are listed at 31.7 tall 10.7"sec N/A tread

I plan on doing the tires at time of purchase with dealer.

 

I would imagine the difference in power noticed and/or gained would be incredibly negligible with a drop in size that small.

 

Yeah I would agree......I looked on BFG site for the weights of each but they weren't listed. I can't image it would be very noticeable powerwise but think it would look silly, I think these dang 275/65 look small enough.....

 

I wouldn't go smaller. I have 305/65/18 BFG AT, and I couldn't imagine going smaller than stock.

 

the stock tire size will be fine... as stated there is hardly any difference.
Now, as far as 5800# with 900# tongue weight you're about or just over max tw... you'll need some airbags or helper springs and I would make sure the tires you get are "E" range, e may be a slight overkill but you'll be glad you did it.
You're going to like the transmission it's really smooth (and I have the 355 gears), your 373 gear ratio will pull your trailer nicely. I used to have a diesel and wasn't sure about the f150, but it's plenty of truck especially with this trans.