Big loss of miles on last few tanks of gas can any one help?

[aheroawakend]

I had a short ram custom installed last summer nothing was wrong at all was getting a good 350 plus miles on a tank giving if i babyed her or drove her hard.
Right before I put a custom cai in to replace the short ram 3 weeks before I noticed I only got 260 on that tank.
I didnt think any thing of it maybe I drove it a lil harder then I usually do. The next tank I got 271. After the CAI this tank I am currently at 206 and its at a quarter tank now with those miles I should be at half a tank or so. I could go 2 weeks or maybe a extra day or so with a tank cause my job isnt to far I dont run that much extras.
Last night I did swap out my spark plugs to NGKs platinum's. Now mind you My ion is a 03 first time I swapped out the stock ones they had some gaping and some carbon build up on 2 of the plugs. Now the thing I wanna know could I all of a sudden get bad gas millage out of no where instead of a gradual down turn in my miles?
The only other thing I can think of is maybe a bad o2 sensor but I have no idea on how to check that at all. If any one has any ideas shoot them my way. Im gonna fill her up today do a lil driving and see if I get any miles back to the tank I hope I do. She only has around 47k miles on her and ive been the only driver so just a lil back ground info on the engine and how many owners.

 

[cobalt123]

weird shouldnt have lost that much

 

[ff_drift_lol]

if it's anything caused by a sensor it would throw a mil light.
try cleaning the maf, run injector cleaner and obviously replace plugs as needed.

 

[Wisenos]

injector cleaner... i run 1 bottle every 10-15 tanks... new plug... checked
clean maf... checked
clean TB... checked
and running about 350-400 miles/tank
still need to change fuel filter (after 60k "100 000km")

 

[camopaint0707]

you are driving harder probably cause of the new found power and sound

 

[aheroawakend]

agreed I drove harder when i had the full cai but it started even before that tho. I will try what you guys have mentioned about the injectors and tb thanks. I wanna get this straight before I do any other upgrades I just got my tires rims and projector hids. Ill post pics in a few days Im busy all this week.