I used this website to find magic gears for one of my bikes. It works and is accurate BUT it appears to be backwards. On my Jamis which has 17.12" chainstays, it says 38x17 falls a little below a 17.07" chainstay length and 34x17 falls above 17.07. This should indicate that both will be too short for my chain stays although 34x17 should be closer than 38x17. Well 38x17 was a tad too short but I ran it tight anyways. About 150 miles later I put on a 34 front and used the same 17 rear as an experiment and it was so perfect I about fainted lol. I mean you couldn't get anymore perfect. This chain had stretched a tad but not a lot and it had just a small amount of droop. How could this be when according to the chart 34x17 still should have been a tiny amount too short? Now if you select use centimeters on their html chart it changes the 17.12 to 25.4 and the results are a mirror image. The same combos are the same distance off the chainstay centerline EXCEPT they are now showing on the OPPOSITE side of the highlighted area. How could this be when the scale never changes (The stay length still gets longer as you go from left to right). Try it for yourself using my measurements and see what I am talking about. http://eehouse.org/fixin/fixmeup.php