I have two Fitzgerald Gliders. One "A" has 355's and a 13 speed. "B" has 336's and a 10 speed. "A" consistently gets better MPG than "B". However, a lot depends on speed and weight. Remember these are 12.7 litre engines. They were designed with 1400 rpm as the sweet spot. If you are heavy, you need higher RPM and 355's are a better choice. As mentioned, the sweet spot on the Series 60 is 1400 RPM. With LP22.5 and a 13 speed the 355 will put you at about 1350 RPM at 60 MPH and you will probably get 7 MPG + if the weight is not too high. Using a 342 would put you at 1300 RPM and unless running VERY light the engine will not be a peak efficiency.