RE: Bad gas mileage on 03 blazer
Take the numbers that are cited on the sticker and subtract about 2mpg average from then to get closer to your actual mileage.
Fuel mileage depends on a lot of inter-related items. The lead in your brothre-in-law's foot, condition of the transmission, condition of the air filter and fuel filter. At this point I would get an aftermarket washable/dry filter from Airraid maybe even the dealer performance parts list since they now carry a dry filter similar/OEMed from AirRaid technologies.
I would suggest replacing the air filter and fuel filter. Air with a dropin aftermarket and fuel with a Wix. The fuel filter should be in the frame near drivers door area, there is a hex fitting on the filter and another on the line since I could not find the correct size wrenches I used a small adjustable for the line and a larger for the filter. Make sure you spray the fitting with a rust penetrating chemical, let sit for 30-60 min and then attempt to replace the filter. First thing to do is to release the pressure at the shrader valve (located near the vacuum brake booster near the firewall), remove the black plastic cap and there is a small area in the center to press and release the pressure (wrap a rag/towel to catch the fuel that will come out of the valve). Open the cap on the gas tank to release pressure on that side of the filter. With the pressure release then you can loosenthe bolt holding the filter in place so that when you get the fitting end loose you can remove the filter. After you get the filter out of the holder then you can get at the plastic sqeaze fitting easier, just compress the two tabs and then pull the filter out of the line. To install is just the reverse of what you did to get it out.