Brand of fuel, winter and summer grades all can make a small difference but 2 mpg is a lot.You would need to look at fuel trim figures on a scan tool to see if long term fuel trims are out of spec but not to the point of setting a code. Is the Chevy high mileage engine would have less friction but the injectors may wear. Are the air filter and spark plugs fresh?
air filter is fresh, spark plugs are not,90000 miles,just cleaned throttle body thought that was the problem was idling roughly and stalling. IT didn't drop all at once but when I got it I was getting 16mpg and it is down to 14 sometimes less. Usually on a trip on the Interstate I could get 18,now I don't see a difference. I was wondering has the quality of fuel gone down that much and more ethanol in the fuel. Thank you for your time.
you might just have carbon build up from poor quality gas, have you tried a fuel treatment, i recomend zmax. awsom stuff helped me with my impalla as it also had a 2mpg drop, and its for both fuel and oil. its about $30 at any parts store.