What makes a car burn lots of gas - 1998 Oldsmobile 88

your O2 sensor must be out. It is in charge of reading you fumes output to determine if you are running too much or too little gas. When this goes out, your car will go in default mode and push more gas in. In return, your car will burn more gas

how much, a lot means nothing, to me they all do. so...???
MPG? is what.

or liters per 100km (metric) ROTW

What exactly are the codes.

well 3.8 engine is not suppose to burn much gas.check for low tire pressure.if engine got many miles on it it will burn more gas,if you live in city making short stops burn more gas,if you driving a lot of roads with steep hills and mountains car will burn more gas.lead foot quick acceleration speed driving burn lots of gas.

Try running fuel injector cleaner through the gas tank. A fuel injector might be stuck open, letting lots of fuel into the engine. Also try replacing your fuel pressure regulator, and inspect the fuel tank return lines.
Good luck
Matt

First make sure you didnt damage any of your plugs by cracking them during installation and second make sure you have the right firing order you may have crossed wires while installing your plugs.

make sure your tires are all at the correct psi, vehicle is tuned up and oil changed deff. make sure the air filter is good

might need a tune up

You don't have a simple problem. Engine light has to be read by a scanner (OBDII) available at autoparts store. Or take to mechanic. After you get code you'll have to be a mechanic to fix problems. Burns lots of gas can be many different causes. Miss in motor, same. Please take to professional if this is beyond your knowledge. Good luck!

Take your foot of the gas. Fast starts ie speeding away from stops and starts burns more fuel.
Make sure your air filter is clean to allow proper fuel air mix ratio. If dirty replace it.
Lighten your load.Don't carry around a lot of dead weight in your truck. The heavier the load the more gas used.
Use a 91 or higher octane gas, this gas will burn hotter and give you more power thus again needing to step on the throttle less to create momentum
Try to maintain speed limit. speeding cost more.