The ratio that is on the transfer case is a 272:1, which is standard for a New Process 231 t-case.
This number however is not the ratio of the gears in your rears. A stock YJ with a 4cyl motor will have 4.10:1 gears in it. Switching these to 4.88's will definately improve but dont expect miracles for gas mileage.
You stated you were getting 13mpg...thats about average. I have seen them as low as in the single digits for gas mileage, depending on the terrain that you drive in and how heavy your right foot is. Even with the new gears, I wouldn't look for more than a few miles per gallon increase.
And the cost offset can never be regained from better mileage. Besides, have you re-adjusted your figures for actual miles traveled vs. miles on the odometer? With the larger tires, when you are going 49, you are actually going 55, which is about a 10 % difference, so your mileage just went up to a 14.3mpg, and that is in the parameters for a stock 4cyl motor.
If you are worried about mpg, buy a rice grinder car, you have a Jeep, its about as aerodynamic as a brick, sooooo, fuel mileage is not going to be a high number. If you really want to keep the Jeep and try and get decent mpg, go back to a stock tire....215/75R/15....they are about 5 inches shorter than the 33's and your fuel mileage will come back up. Unfortunately you cant have both, big tires and high numbers in the MPG department.