I brought my 2015 CX-5 grand touring 2.5 eng. to dealer in No.Jersey for a routine oil change. Used car few days later. Hesitant to shifting & when pulled out on highway, huge clouds of Grey/white smoke out of exhaust behind me. Slowly drove car home. Thinking I blew head gasket, I Checked dip stick.Oil was All the Way Up Stick with no signs of water! Nor under oil cap. Anti freeze level was a bit low, but no signs of oil. Called Dealer said to bring car in most likely minor problem & not blown head gasket since car just reaching 48k mi. They Immediately changed oil & filter upon arrival.Service rep. kept car overnight to test out himself giving me loaner car. Next day I get call, pickup,your car is good. Immediately noticed Average Miles Per Gallon only 18! This car was good for Average 30 with A/C on. I had them put in writing (my request) they over filled oil & that I suspect Cat Or Sensors are probably contaminated & would prematurely create future problems. The dealer on about 4 return visits since, has changed plugs, compression test 145 across board. Replaced sensor up front for air flow. During all this never an engine light on! I still suspect lazy O2 sensor or Cat bad! At best MPG 24 with mostly hwy. They will not replace components unless Engine light or codes present. Any Suggestions? What other courses of action could I take? BTW my extended warranty ends in Nov 2022.