Why dose my jimmy 4x4 used so much gas

Like the above post says, it is dependent on a lot of variables. How much weight do you carry? Do you have a roof rack? Are your tires almost worn out or under inflated? It's surprisingly interesting to find that a roof rack can cause you a few MPG from the turbulence. Worn out or under inflated tires create a lot of rolling resistance and drop MPG. Are you carrying a few old refrigerators or tire chains inside the vehicle? If there was a misfire or some other subsystem that was causing lowered MPG, the CEL would be lit. If it IS lit - those older computer systems can cause some serious MPG drops because they go into 'punishment mode' to get you to find out what the ECM 'sees' as bad. So a bad plug = CEL. A bad plug wire = CEL A burnt valve = CEL A clogged air filter = CEL Slow reporting O2 sensors = CEL STUCK O2 sensor(s) also = CEL Many early systems won't collect data AFTER the CAT, so you may not see a CAT failure. It's worth testing for a back pressure problem, but don't go changing too much as the factory has much larger computers than the average muffler shop - on how to maximize MPG. The idea that back pressure can cause loss of MPG is rife with possibilities. A partly clogged Catalytic Convertor CAN drop performance and therefor also MPG - but it would pop that CEL immediately! Your MAF or BMAP sensor would 'see' the problem and that = CEL. Your TPS voltage compared to the ECM's profile information - would be out-of-sight so that = CEL. I changed my exhaust system a few times on my non injected, ECM'd '89 K5, and the best fuel MPG was with a single 2 1/4" system, blending both the right and left bank into a Y collector and then to the smaller OD exhaust system. SIDEBAR: yes - i always keep my emission controls up-n-running at all times. :) The back pressure was ridiculously HIGH, but so was my MPG - although power was down. On the same hand, a wide-open exhaust system resulted in a very serious drop in MPG although power was as high as the engine could produce with a mild Crane cam and Edelbrock Lo-rise manifold and Q-Jet carb. The noise level (dBs) was out-of-sight too. As for the K&N filters - they are snake oil. If you have an ECM (you do) the so-called increase in flow and less restriction is phony and won't even make any difference other than making the life of your MAP a lot shorter. Think of it this way: if the ECM 'sees' too much air-to-fuel ratio, it will increase the injection time and duration to compensate anyway. Why even bother to increase air flow if the ECM will also turn UP the fuel to compensate? You gain nothing but a large $$ for that K&N. I worked almost exclusively on GM/Ford/IH/CAT/John Deere/VW and lots of other diesels in the early '80s, and I saw how the clean air side of the turbines were worn down to nubs with K&N filters. They just pass too much crappola .... and that dirt.sand/bird droppings etc will sand blast the turbo fins to nubs after a while. Don't waste your money on those K&Ns. [/soapbox] HTH -