86 fz600 in a 550 maxim

I have a 86 fz600 that is in the frame of a 550 maxim. thus far everything has been pretty smooth, it runs great and is surprisingly reliable for a frankenstein. But the issue that I have is I don't know what size the rear sprocket is supposed to be. Its also getting really poor mileage and I think it might be because the rear sprockett is too big, but like I said earlier does anyone know how big that sprockett is supposed to be or any other ideas on fuel efficiency. to clarify the bike acts like the clutch is stuck around 45 to 60 mph.
thanx eddie

Have you messed with the carbs? If the floats are too high, it'll spew fuel right into the throat(s), and I can tell ya from experience you'll get horrible mileage.

IIRC, my friend's XJ550 had a 48T rear sprocket, but if you've got the engine and front sprocket of one bike, and the rear tire of a different bike, I wouldn't place much stock in that being right. Unless you're waaaaaay off, I doubt it would have much effect on MPG anyway.