I wrote about setting the mix on a SPICA in another post. I have now driven far enough to see the change in gas mileage (or whatever one calls it in countries with metric) and it's considerable. With the mix set as carefully as I could with no load on the engine, using a digital tach to see the revs, I was getting 20 MPG in rural driving (not much stop and go, but not highway). After setting it under load, 24 MPG. 20% difference.