Hey chris
Yeah, changing the tooth does slow it down a bit in the city. I've put about 500km on them since the first post and I realize that it does definately change it more then I originally thought. If I know I'm going to be driving in the city for awhile I'll probably change back to the one tooth smaller sprocket. I definately agree that sprocket size is obviously personal preference, but I'm glad I tried out the change.
Hey fergie
I *believe* it is an 18, yes. I can't remember exactly, but I DO remember that it is one tooth larger then the original. Also, it is the stock CB750 front sprocket for 1978, it fits the 550 fine. It's pretty close, but I could turn it no problem and there was no scraping. As I said above, I just finished about 500km on this setup and still am happy I went to that for highway driving. I think for city driving, I'd go down one. Even two down from my present set-up (one down from stock) woudl be neat - it would be a very quick bike off the line and below highway speeds.
Cheers all
Barry