Tried search. Seems like there is little information out there on the newer Cannondale serial numbers. I'm putting my 1998 R1000 up for sale, and am not sure if it's a 58 or 60cm frame (I'm also trying to figure out what size cyclocross frame to get, making things more pressing).
I've got this: http://www.vintagecannondale.com/info.html, but w/ all those numbers, Cdale doesn't put the frame size in that serial? Any hints on this? I've taken frame measurements, but they put it at a 59 cm frame. And my tape measure measurements are far from accurate.
I get 57.2 and 81.2 vs. Cannondale's 57.5 and 82.2 for the 58cm, so I'm guessing it is a 58cm. But the tolerances are such that depending on where I'm measuring, it could easily be off (I'm using a tape measure and converting to cm).
I think you have a 58. There's nothing really 58 about the seat tube on a Cannondale 58. Measure the seat tube from the center of the BB to the top of the top tube. Also measure to the top of the seat tube (these two are detailed on page 94 of the catalog).
There's a 1 cm (or almost 1/2") difference between the 58 and 59. Mark the difference on your tape measure and it should be apparent which size you have. And I don't know if the 59s were commonly available. I think that's about the time they offered custom framesets.
Finally, if the bike fits, just use its dimensions to buy your next bike/frame. What's more important than the label is the actual size.