Why does my 91 oldsmobile cutlass supreme get 10 miles to the gallon?

My 91 oldsmobile cutlass supreme is a 3.1 v6 and only gets 10 miles to the gallon. I changed all 6 spark plugs and wires, changed the fuel filter, and straight piped the catty. They say the o2 sensor is fine but I'm not convinced. Any ideas?|||Keep all emision controls stock and hooked up and she should get great gas mileage.|||Could be somethin else wrong with the ignition system. Ignition timing might be off. And yeah, driving habits make a difference but shouldn't make that big of one. I suppose the o2 sensor could be part of the problem. Sounds like it's running rich. If you had to hollow the cat out, you're probably running rich. Could be a clogged vacuum line making you run rich. And "A+", cleaning the cat out inproves your mileage! That emissions crap makes your mileage worse and robs horsepower.|||driving habits also can play a role on that, such as quick acceleration and braking.