It couldn't hurt. If you crunch the numbers, your truck had about 31's on it before and now 33's. That is 2 inches in diameter, which means your truck goes 6 inches (2 inches*pi) further on each rotation they make. Lets say 31's go 93 inches on one rotation, this means 33's go 99 inches. The 33 will go around 640 times per mile and the 31 will go around 688 times per mile. This means your speedometer is off by about 7%. So really, If you drive a mile on your odometer, you really go 1.07 miles. This means your actual fuel mileage is 12.8 to 16 mpg.
The real problem is the amount of weight you've added with the 33's. You probably added about 80lbs of wheel and tire on top of the stock wheel and tire weight that the your engine now has to spin. Thats going to be the real culprit. Try and ease up on the skinny pedal too, that can save tons of fuel!