That's a huge gain in mpg for a tiny piece of plastic, doesn't seem possible in my experience.
For me, the prodrive lip made things a little less twitchy. Using w/ an APR splitter made the front end so grounded, the rear felt terrible
. Gas mileage dropped with that setup.
V2 lip made about the same difference in reducing twitch at speed, but no noticeable change in mileage.