Take 230 and divide it by how many gallons of gas you put in to get your MPG. I've got a 96 trooper and I only get 13 MPG with the stock tires. I just put on some 32's so I don't want to know what I'm going to get on my next tank of gas.
You should get about 14 MPG or so with 31" tires.

/1996 Trooper SE\
OME 912's w/2.5" spacers, Superwinch hubs, 265/75R16 Yokohama Geolanders (street), 34x10.50x15 Super Swamper LTB's, Indy4x HD Tie Rods, Indy4x Rock Sliders