My buddy has a 2003 Trailblazer with 64,000 miles. has the 4.2 straight 6. he has had it about 6 months. when he first got it, he was averaging about 23 mpg on the highway. but now he is only getting 14 mpg or less. Truck runs fine. when put into nuetral, the truck coasts fine, so sticky brakes arent an issue. Clean air filter, tranny shifts smoothly and stays in correct gear. He had 18" rims with larger tires, but he put his stockers back on to try to gain some gas milage, but it didnt make a differance, also took off aftermarket intake and put stocker back on which didnt make a differance. there seems to be no problem with the motor/trans. Idles great, doesnt serge, tranny shifts great. Im thinking a sensor. MAF?

Take out the plugs and inspect them. If it'* running rich the plugs will be dark. Also check the tail pipe, is it black inside? Is he taking in to account winter blend fuel, warming up the engine etc.

is started doing this several months ago... the fuel milage dropped after he went to larger tires and rims. but when he changed back to stockers the gas milage stayed at 14 mpg or worse

I would check the plugs. He may have run it rich and the O2 sensor is getting tired.

The problem steve is when you put bigger wheels and tires on a newer vehicle the computer figures it out and essentially you are dropping your gear ratio..In turn to fool the computer to thinking all is back to complete stock you have to swap rear gear set

Also not trying to be rude but if he didnt beat the **** out of it he may not be having issues...we are talking about ole boy who floors it during shifts right??