why would this be happening!?

ok so today i got my car back from the dealership, they replaced the cat and replaced the rear spark plugs at NO charge to me at all...so now my gas mileage is move up n down like crazy... earlier today it started out at 22.1 then 22.2 then gradually went down but whatever number its at its always plus or minus .2 or .1, is it cuz of the new sparks...i mean im completely clueless...lol right now its at 17.1 ten itll go to 17.3 i mean b4 i had ne thing replaced it would always stay at one number like 22.2 for example....?!?!?!? im lost...lol

Well, when I had my cat replaced by the dealership, I had a few problems with smells and gas mileage and then the engine light came on a day after the cat was replaced.

The problem was that I needed a rear o2 sensor for the cat that was replaced under warranty and that solved my problems and the car been running great ever since then.

So maybe you need a new o2 sensor.

They reset the DIC right? It's still averaging out your gas and it will vary a lot until it's had a lot of miles driven on it.

ok thats what i thought it was rook btu i wasnt sure, so how many miles do u think itll be before it actually stays at one number lol...ya Dre im getting some weird smells too from my car but idk how to explain em....lol but could it also be the new spark plugs being worn in? lol

I have driven my car over 200 miles and it's still shifting the MPG around.

damn...lol

Its going to shift around for a while. If you reset it yourself it will jump from 50.0 to 4.5 to 25.6 etc.

Its just an average, and without a lot of data its going to vary a lot

oooo lol thats good to know cuz i was like now what the hell uis wrong...lol