Well based on a guestimation, my fuel ecomomy has (THANKFULLY) improved. Mike said his car computer registered 2 mpg improvement. That would mean my average of 17.5 is closer to 19-20. Well I will say that my car now is getting near if not over 430 miles per tank, maybe more but hard for me to discern as I'm not going to run my car to it dies from no fuel.... HOWEVER, the added zip will also make your fuel mpg worse. If you are going to tenderfoot it everywhere then you can expect the added mpg. Hard to do with a vx.