The 235 tire is basically a 29" tire (Metric to inches), which makes them 7.46% smaller than the 31's. Unlike driving larger tires where you add miles, you will need to deduct 7.5 miles per 100 driven. This is assuming that your speedo is correct and that you have 4.56's and not 4.10's, not all v-6 5-spds came equipped with 4.56 gears, only those that came with the 31" tire package did which would be identifiable by the axle code G254. You can check your speedo/odo with a GPS or the mile markings along the freeway.

Were your mileage gains before/after or with the 235's. If before your mileage gains would be 231.3 instead of 275, and 277.6-300.7 instead of 300-325. The 231.3 to 278-201 is still a good increase, about a 23% increase, or from 13mpg's to 18mpg's. The 18 is about right for combined driving, I average about 15-17 depending on throttle use with my heavier rig. I would be curios to see what I would get if I remove about 800lbs of weight.