Cannondales aren't known for generous tire clearance, so I highly doubt a 42 would fit. 37 maybe but probably with pretty limited clearance.Red Horn said:Have decided 37's would work out OK too. Will they fit?
Only way to know for sure is to try it, either at the LBS or eat the cost of return shipping if you order on line and they don't fit.
To get a rough idea, take a caliper and measure the inside of your stays where the widest part or your current tires are. You'll probably want 3-4mm of clearance on each side (more if you're riding in mud). Any less and you'll get some rub on the stays with tire/tube imperfections, mud, or flex under heavy pedaling.
However, note that most tires don't measure out at claimed sizes, so even if you measure your stays, or somebody posts that they fit a 37 on that frame, it doesn't mean squat unless they are running the same tire. Some brands run way small, others run large, but I've not found many that run true to the size listed on the sidewall.