Okay so me and my wife have four mirages, and we deliver pizza in them. 2020 ES 39mpg city and 46mpg hwy, 2020 LE 37mpg city 44mpg hwy, 2014 DE manual 41mpg city 48mpg hwy, and we just bought a 2015 ES with 105,000 miles and it is getting 28mpg city and 46mpg hwy, I compared the fuel trim and o2 sensor voltage to the other three cars and it is very similar. fuel trim long and short is within 4.0% and voltage on o2 sensors is within specification according to FSM. I changed the plugs and the belt, have done italian tune up and made sure tires and brakes were not the problem. Alternator tested fine. I tried turning the blower fan up to 5 bars instead of 1 and it got 35mpg city, so thought it may be a/c, tried on 5 bars again last night and back down to 28mpg. It has me stumped, unlike the 2020 mirages i can not read the fuel pounds per minute on the 2014 and 2015 so I don't know how much is uses at idle. my 2020ES is .01lbs/min @ 750rpm and the 2020LE is .02lbs/min at 750rpm, not sure why they are different but the LE does get worse mileage than the ES.

So main question is, what can I check next on this 2015 ES to determine why its only 28mpg

my secondary question is why are my 2020 mirages using different amounts of fuel?