I have the Quadzilla mileage max and I was concerned whether or not it increased the rail pressure which I didn't want so I contacted Quadzilla. They said it only does the timing. I found it helped my mileage by 1-2 mpg but even that is subjective since my driving is different from day to day. My truck has always gotten very good mileage and this helped I think. When I started having starting issues I took it off as part of the troubleshooting to see if it had any effect on it (which it didn't) but I still haven't hooked it back up.