On paper, the G55 gets better gas mileage. My '02 G500 averaged around 13mpg. My '05 G55 averages around 11.5. That doesn't sound like a huge difference, but it adds up real fa$t. There's one thing to remember, we don't own a G55 (or even a G500) because it's economical. It's not. We own them because they are the bad-est things on the road... correction... planet.