I have a 1999 catera and I have noticed that over the past month or so the gas mileage is getting really bad. I had it into the garage and they changed the air filter and plugs and it still is bad. I go 50 to 60 klms on a 1/2 a tank of gas and it cost me $32 to fill it up again. Can anyone give me any idea's what else they sholud check. Any help would be greatly appreciated.