when you install the speedo cable it might be a good time to make sure you've got the right speedo pinion for the tire.
3.73 w/ 26" is a 38 (with a 27" is 36)
here's a wild theory though... is your tach the type that has multiple cyl settings and maybe it got bumped to 4 or 6 cyl and you're getting a 4K reading when you're really turning something less than that?
because, again, the math has me scratching my head. even at 3K you'd be clipping darn near 65mph