Actually tested this in a semi controlled environment.
2002 VW Jetta. 4 holer. 23K miles.
DTW (Detroit Metro Airport) to ORD (Chicago O'Hare).
Without 27.4 mpg.
With 29.3 MPG.
New Plugs, Oil, Oil and air filters before 1st trip/test.
Same car, same miles, same driver, same load, even real close to the same temp out, there and back.
1.9 mpg increase.
Mixture: 1 oz to 6 gallons.
Terry Bogusz
GSE Manager
Spitit Airlines
Detroit Metro Airport