[rich weyand]

I'm assuming this is a 2WD truck, or tire considerations get very different.

If you run 15x8s all around, you can rotate tires to even out wear, so that all the tires need replacing at the same time.

P245/60R15 (the BFG Radial T/A is the only tire tirerack has in that size) are only 26.5" diameter, instead of the stock 29", so on the rear that would 1) give you effectively a lower rear-end ratio, 2) worse mileage, 3) require you to dink around with the speedometer drive gears to get the speedo to read right. Now, if the truck has a 3.23 or 2.73 axle ratio, or something like that, that would be an improvement. If it has a 3.73 right now, it would effectively be increased to a 4.08, which could be a little high, depending on the driving. Lots of long distances in Texas; you might want more of a highway ratio than that. On the front of course, it wouldn't matter.

Same thing with P295/50/15. 26.7" diameter.

One of the stock tires is a P235/75R15, which is pretty tall and won't mount up to an 8" rim.

P255/70R15 might be interesting. 29" diameter, so no speedometer drive messing around, mounts to 8" rim.

P255/70R15 on the back and P245/60R15 on the front would give you same ratio/speedometer, both mount on 8" rims, and would give you about 1-1/4" of rake (drop in the front relative to the back).