I had a base model 02 cavalier that almost always got 400 miles to tank or more even when I wasn't easy on it. Now I have an 02 cavalier LS and it struggles to get 300 a tank anymore and I've tried about everything besides replacing the map sensor. Only difference is 1"bigger rims and PW/PL. Fuel pump is maybe a year old, water pump is about 6 months old, just did plugs and wires with ac delco parts (didn't change anything, yes I checked the gap). I use fuel injector cleaner occasionally, replaced the throttle body gasket and cleaned it up, replaced the TPS. Replaced heater core myself and replaced coolant reservoir. Higher octane gas also didn't help much. Any suggestions here? Forgot to mention the dealer I got the car from didn't mention it caught on fire at one point where the plastic behind the rear seats is. They replaced the most of the wiring harness but a section still looks burnt, never had any problems though.

Also when I pulled the plugs they showed the engine is running a bit hot but it's never overheated.

You can try one step colder plug and see if that helps. Do you know if the ECM enriches the mixture, if it detects warmer than normal engine temperatures? I don't know anything about OBD2. Don't know if the LS has a more aggressive tune than a base model.

I suppose the tanks are the same size.

Are your tires the same outer diameter as the factory size? If it is under diameter, you will decrease fuel economy a little. Over size should increase it slightly. I know you said the wheels are 1" larger, but if the tires are very low profile, the diameter might be less than factory. If the tires are wider than factory, that could decrease mileage some as well.

Is it properly aligned?
All hubs good and no extra drag on anything?

Power windows and locks don't add very much weight. Certainly not enough to cause a 33% loss of fuel economy.

Just throwing out some ideas. Hopefully someone with more 3rd gen experience will know why you are see such a difference.

Tires are recommend size for the LS, I've had it aligned a few times and it's never quite write I've taken it back multiple times. It's never been right on the 3 j body's I've owned and I've tired several different places but I won't go tot a dealer ( too much $ for an old ass car). Hubs have also always been an issue for me even tho I use Moog or Timkin hubs and have then replaced in pairs usually, at this point tho they been fine for a while. I get some trims and live data on my OBD2 scanner but nothing looks out of the ordinary. Forgot to mention I now have a cai on rksport but it was having bad mpg before I put in on. It was great when I first go the car but slowly got worse and won't come back and I don't drive hard usually just cruzing.

Did you mention whether this is an automatic or manual trans car? If it's an automatic, is it going into overdrive? If a manual trans, then is the clutch slipping at all? I have an '01 base model and I typically get about 370 miles out of a tank, which works out to about 29 mpg. I consider this pretty good since the car has over 127k miles.

It's an auto, yes it's going in into OD the RPM's arent high or anything when I'm on the interstare doing 80 is around 2900rpm iirc. The one thing I forgot to say is I dropped my tranny pan and refiled it twice because I didn't want to drain the torque converter. Got a got an delco filter and Valvoline synthetic ATF and got a Haynes manual since it doesn't have a dipstick. Lordy that was a pain lol. The tranny makes a sight groan noise if you listen but it's been going it for over a year if it was going to break it should have already.

Did you ever have any success in solving your problem? If you did, then what was the solution? My other thought was that the ECM was getting a low engine temp signal which would enrich the mixture. This could be due to a failing coolant temp switch. The O2 sensor could also be going bad, which directly affects the fuel mixture.