[brucepick]

I'd say you're right on both concepts. That is, reducing engine rpms and getting a bit of a wheel skirt effect. You'll see some small fuel-thrifty cars where they line up the tire sidewall with the body work in an attempt to get a smoothed air flow.

You could consider changing the tranny or rear end gearing instead of changing the tire diameter. I don't know what the options are for your truck but there are usually several gear combinations out there to be had. The cost might be in the same range as a set of tires.