[Christ]

S-10's are great little trucks, honestly. Being carb'd makes it easy to work on, repair, and adjust. The first things you should look into are status of the tune-up, and making sure the carb is properly tuned and adjusted. The truck, by all rights, SHOULD run a bit lean. The 2.5 "Iron Duke" doesn't like too much fuel, from what I've seen of them.

Also, that whole engine is Iron... it can "take the heat", if you know what I mean.

If you're serious about modding, don't think about transmission swaps. They're all the same. You could get a V6 rearend, though, which (IIRC) has a longer ratio.

Beyond that, if you like style, Camaro and Corvette are the only OEM wheel options for it (and the variants thereof). After those, it's aftermarket or wheel adapters.

There is a huge aftermarket available for S-10/S-15, but not much of it benefits MPG or aero, so you'll have to pick and choose for what's important to you.