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.

the truck came with 235's, and it has 4.10 gears. the problem that i have is that before i got the truck, someone else put in a speedo cable for a truck with 31s. i want to upgrade to 31s, but dont have the money.

The issue with the speedo will be more gears then a cable, IIRC. I dind't think there was such a thing as a spedo cable for 31" tires???

ok. so to get the speedo on the money, id have to change the gears? im a little confused.

To change the speedo gears is not too hard or expensive, I think, but most people don't bother? I'm running 31's with the stock gears that came with my used transmission. My speedo reads about 10 mph under the true speed. In other words, when my speedo reads 60 mph I'm actually going 70 mph. I live with that with no loss of quality of life.

It'll actually be off by a percentage.... <img src="/forums/images/graemlins/pfft.gif" alt="" />

This shoudn't effect your speedo at all, unless the PO actually changed the gear at the back of the speedometer, which I am not sure it can be done that way with the Yota's. Changing teh cable alone wouldn't/shouldn't have had any effect on the speedo reading. Now if the PO changed the diffs, than it would through the speedo off by as much as it is.

To determine what gears you have, you can lift a rear tire, mark a spot on the driveline, and see how many revolutions of the d-line it takes to turn the tire one full turn. If you have 4.10's it will take 4 full turns and about a 1/10, if you have 4.56's it should be about 4-1/2 turns.

Back to the MPG with the V6 motor.
Last time i compensated for my 4.10's and 35" tsl's I got 14.5 mpg mixed highway and city driving. Good enough IMO for under geared. Ill be better when im geared to 5.29.