In general, thinner tires should provide better fuel economy since they're lighter and provide less rolling resistance. They also help with hydroplaning since there's less water to evacuate under the tire. Wider tires should do better in soft surfaces like mud and sand since they distribute the weight over a greater surface. In a perfect world, there should be no difference in traction and grip between a thin and wide tire on a flat and hard surface since grip is just a matter of friction, which is dependent on coefficient of friction and downward force and has nothing to do with surface area. But we're not in a perfect world, so in practice, a wider tire should give you better traction on hard surfaces as well. But given how relatively miniscule the weight savings is from a 12.5 to an 11.5 wide tire, you probably won't see much acceleration or economy improvements.

 

Also keep in mind the weight of the tire you are buying, it should help going to a narrow tire but the tire you choose is probably more important. For example Toyo M/T are around 80lbs for a 35/12.5r18 and BFGoodrich KM2 are around 67lbs for 35/12.5r18. The weight of the tire will make a difference as well. The rolling resistance of a M/T tire vs a A/T is something to keep in mind as well.

 

Narrower tires do better in mud/snow/ ice usually. The PSI is higher on them due to the less contact patch which is what gives you bite on snow/ice. Mud its good because it cuts down threw to something solid. Basically wide tires are good for floatation if you cant hit the bottom, narrower tires are better if you can get to something solid (for offroading)

 

I know it"s not the same engine but in my old 5.3 I went from a 33" x 12.5 mud grappler to a procomp mud tire 11" wide. And I couldn't tell any difference in performance and MPG. But it looked like crap.

 

The wider the tire the more they stick out and throw stuff on the side of your truck.

 

I'm right up there with ya on this. I love the thought of a 305 or 315 from the dry traction standpoint, but from the MPG standpoint, even a 295 seems aggressive and expensive.

Sure, more tire will LOOK great, but ultimately my truck spends about 99% of it's life on the paved roads, and looks can get expensive!
An MT will also look better than a similar AT tire, but as mentioned above it'll weigh more too (normally)

 

12.5 = looks bad a$$, and you can take the floating approach off road. A 12.5 aired down on sand, lose ground, deep snow, and in some cases mud, makes it feel like you are on a super hiway smoth and loads of traction with little rolling resistance. Rain and a dusting of snow to a few inches you get exactly the opposite.

Skiny tires: better for rain, shallow snow, the right mud pit. (Some pits you want to dig into good ground, others its best to stay high so you dont do any pumpkin draging)mpg, less paint chips, less stress on drive train, cheeper tires.

If 35 x12.5 did not cost twice as much as 33 11.5 id still be on a 12.5. Other then tire cost i have mpg, and not having to be as cautious in the rain to be grateful for. But thats about it for me. Every thing else is a compromise for me.

 

The skinner the tire the lighter the tire, thus less rotating mass to get moving

 

I don't ever plan on lifting/leveling my truck. But, I do plan on new tires. I like the Cooper AT3 on my ranger, but my favorites were the Bridgestone REVO2's. I got 80,000 miles on one set. They're just expensive as fark.($1400 for 31x10.50 15's before mount/balance, tax) I've ridden in many a lifted truck, and all have their issues. I just can't justify sacrificing all-around performance for something that looks cool. Plus, if these Ford's are anything like their predecessors. They'll burn through more front-end parts than normal, and this coming from someone who likes Ford. If I lifted my ranger, I'd go through ball joints nearly once a year. The new truck I have no idea, nor do I care to find out.

That said, I think many of the trucks on here look awesome with big tires. It's just not my cup of tea. Keep modding those rides, and maybe I'll become a believer someday.