Keep your tires properly inflated and you'll get better mileage. Only way to measure mileage is on a dyno. Driving/traffic/environmental factors are too complex to DIY. I calc my mileage every tank and 2 mpg is statistically insignificant.
I'M ALL FOR AERO AND HOPE THEY DO WELL BUT I DONT USUALLY TRUST AUTOMOTIVE CLAIMS THAT THEY ADVERTISE WITH THEIR PRODUCTS,EVEN DYNO CHARTS AND MPG STATISTICS CAN BE MANIPULATED TO MAKE IT LOOK BETTER THAN IT REALLY IS