I also had a 99 MXZ 600 and a 02 Legend 600 and both sleds were in the 11-14mpg range. That said I don't know what BRP did to the 500SS series but my 06 model consistantly returns very close to the 20mpg mark, ridden responsibly on the trails.I don't know where you guys are getting figures that say the 500SS is getting better mileage than the 600 SDI.
I had a '01 MXZ600 that would get around 16 mpg (Canadian). My '04 600 SDI (in a ZX chassis) gets 21 to 23 mpg, as does RevIAm's '04 GSX 600SDI. My sister in law w/ her '04 Legend 500SS gets around 16 mpg.
If the 600SDI you're comparing to can't get that kind of mileage, something must be wrong w/ it.[/b]