Well ran a tankful, MPG difference came out to be 0.4ish MPG. Its no miracle juice, but it seemed to have helped a little. Observations so far are
Starting up after sitting overnight - no more pig rich smell in exhaust
Quieter at cruise and at idle, yet idle speed has no change
Improved throttle response, before it would bog then go, now it just goes
What I thought was a sticky lifter noise seems to have quieted, could have been a dirty injector.
On a certain stretch of freeway quarter throttle would barely hold me at 65, now it inches up to 67ish
If I floor it hard then let off the gas and drop to neutral it will rev up to 1250rpm. Then tach and spedo match needle speed as I slow down
I did give it a try last fall and I got improvement of around 8% at regular speed. But this spring I gave it a second test at low speed (hypermiling or HE) and I saw no improvement. I drive a small Smart cdi 800 cc dsl. See this thread :