[scoobytuff]

I would say to start with a good tune up. Plugs, distributor cap, rotor, spark plug wires, air filter and an oil change. Cleaning the throttle body won't hurt anything either. O2 sensors are usually pretty cheap and If it were me I would change that also. I'm not sure what kind of transmission the truck has, but while your doing a tune up would be a good time to service the transmission. It sounds like a lot of work and it really is. The more of this stuff you do in the beginning the easier it will be to keep track of in the future. All of this service stuff out of the way get him a fresh start and a good baseline on where the truck stands mechanically. I've even seen brakes that will slightly drag and have some horrendous effects on fuel economy. On the transmission part of it a slipping clutch in a manual transmission can cause bad gas mileage, and in an automatic transmission look at the kickdown cable adjustment. Really without going to far into it mechanically, get the thing a good tune up! You just can't go wrong there anyway you look at it.