[NorCal 4x4]

Toyo M/T 33x12.5R18LT 33.0" diameter 75 lbs.
Toyo AT LT325/65R18 121R D/8 34.5" dia. 75 lbs.

As far as gas mileage, since they are the same weight, that's a push unless you went with wider/heavier rims for the 325s.The larger diameter of the 325 will raise your overall gearing. In theory this might actually improve your gas mileage, but with a Titan, it can also lead to torque convert not locking when it should, and later shifting, which would use more gas, and the larger diameter gives you less apparent power.

For an 18" rim and a 33" Toyo AT, I'd get the LT285/65R18 125S E/10, only 57 lbs, and 32.5" diameter, or a LT275/70R18 125S E/10, at 55 lbs, and 33.1" diameter.

For the Toyo M/T a LT275/70R18 125P E/10 at 66 lbs. and 33.6" would be better, although still a little heavy.

I have Toyo M/Ts in a 73 lbs. 33.5" diameter and they are definitely not as smooth or as quick at acceleration as my aftermarket, 58 lbs. 32.8" dia. BFG AT KOs were. Plus I lost a little bit over 1 mpg compared to the KOs, (2 mpg compared to stock 44lbs. 33s).