The door says run them at 38psi cold.

 

Don't forget when checking your mileage after installing larger tires, that your odometer readings are no longer accurate. I upgraded from 265/60 18s (30" tire) to 275/70 18s (33" tire). If I used the odometer (without calculated the increased distance travelled due to the increased diameter of the wheel) it showed a substantial loss in mileage. When I calculate the 8.6% increase in actual miles driven, the mileage is basically the same. This point is not valid, however, if you have reprogrammed your speedo. As far as the horsepower loss, there is none - as said above, what you have effectively done is increased your gear ratio of power to the road due to the larger tires.

 

Narrower summer treads will not only be lighter but they will also produce less friction. Try to find one that will fit onto the rim and be the same diameter as your stock wheels.

Go with Toyo Proxies if you can. Then swap them out during wintertime. You're not going to get that great of mileage in the winter anyway.

 

Great point...makes perfect sense just never thought of that. That would explain my perceived loss of gas mileage.

 

I would rather stick with American made tires. ...either Michelin or Goodyear.

 

Michelin are French.

Goodyear would be good if they didn't flop all over the road. I'm a die hard U.S. product man, but I swear the Goodyear mascot must have horns and a pitch fork.

 

Do you travel outside SC in your work? Everytime I've switched from a stock setup the MPG's have gone down. Go back stock and get some straight highway tires that can handle the heat. Your speedo will be correct so you can get accurate readings.

 

The door says to run them at 38. I bumped them up to 39 yesterday. The front still look a bit slack but I'm going to run this for a while. I just changed the air and fuel filter so hopefully that will help with the MPG a little.

 

I'm leaning toward the LTX M/S size P245/60R18 These are about 1% smaller than the stock size where the tires now are 1% larger than factory size. What do you think?

 

So much mis-information.

The tire size upgrade you did is 5% larger, so you are travelling 5% further than your odometer tells you. Figure ~.75 MPG better than your new calculations.

Weight was the problem when you changed tires/wheels. It is much harder to turn a 55 pound tire than a 40 pound tire. Skip the C/D/E rated tires because they are heavier than p-rated for the same tire size. The LT tires tend to last longer, but there are exceptions. The important thing is the actual load rating of the tire. For example, the 245/60R18 has a load rating of 1984# per tire, which is a little low for 1/2 ton truck duty. Check your driver's door sticker for the rear axle rating. You want a tire load rating at least half that, maybe a little more for safety margin especialy p-rated tires.

265/60R18 would probably be a good sie for you. Firestone destination LE is a good value with Goodyear Fortera Silent Armor the longest treadwear.