Remember your odometer is off 6.54% by having 235/75/15 tires vs 205/75/15. Say you get 200 miles on your odometer. Multiply that by 1.0654 to get the new mileage reading and divide that by the amount of fuel you used.
For what it's worth, my '95 16V (2dr) kick with a winch bumper, winch, lift, roof rack, hard top, rock sliders, 235s and MT tires generally has been getting about 19 to 20 MPG.