What could make the gas mileage drop from over 30 mpg highway to less than 20? on 2007 Hyundai Elantra

I had brought my car in for the 60,000 mile required service check and after I got it back the gas mileage was not the same. Asked the service dept. mgr. and was told it was due to the winter weather, gas additives. But winter is over and have been going to better gas stations and it has still not helped. Would like to know if the days of good gas mileage is over or if it is something that could be fixed. I've only taken it to the same dealership service dept and have kept up on all the routine oil changes, maintenance, etc that they recommend. Thanks.

Asked by for the 2007 Hyundai Elantra
is your ck eng light on? have live data scanned to determine if there is a fuel trim issue
Thanks, no check engine light on.
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