When I was commuting on the M4... 120 mile round trips, I tried Millers additive, claimed to increase octane (cetane) by a couple of digits. It resulted in better mpg (see my earlier post). That was on cruising style driving mainly with everyday urban on top. It was consistant improvement. The extra cleaner in the millers I expected to give a flush out and improvement. On returning to additive free the mpg dropped back. I therefore deduced it had an mpg improvement due to octane adjustment.
I haven't used any since retiring and reduced mileage. I may try the VPiwer as a test.
