I am using the Marvel Mystery Oil again, in our 2007 Honda Accord. A while ago I got 38.5 MPG in out 2006 Accord, and a similar MPG number in the 2007 Accord. But now I deflated the tires back to factory-recommended 30-32 psi. Conditions: mostly long trips and about 10% A/C use. I would say about 90% highway at 55-65 mph.
I drove using techniques I outlined in my post on maximizing fuel economy, but with no tire overinflation. I added about 4 oz MMO to fuel (per 15 gallons).
I added the Marvel Mystery Oil (MMO) to the tank and filled it up with regular 87-octane gas. This time I drove 546 miles before refueling and refueled with 14 gallons of gas, averaging 39 MPG. This MMO test included a couple of short trips and about 10% of time I also used A/C. Without those, I probably would be able to get to 40 MPG. And I expect MMO to keep the fuel system clean, even when using semi-questionable (not "top-tier") gas. I will give MMO another try in my 2004 Infniti G35. The previous attempts did not achieve much.
My web site