Yes, and yes.
When I A-B tested a "suite" of aero mods together on the 2014 Mitsu Mirage:
- air dam
- medium kamm back extension
- grille blocks
- 1 wiper arm delete
- passenger outside mirror replacement
- driver's mirror folded back
With all those taken together, I only saw a 4.5% improvement in fuel economy at 90 km/h / 56 mph.
http://ecomodder.com/forum/showthrea...ini-29182.html
That's why I'd be very reluctant to try reading the tea leaves on any single mod from tank-to-tank testing. A gain or loss will be hidden by normal variability.