So I found the instruction video I watched on the Tech 2 setting tire size and it was a freaking Saab.
I went through most of the pages on my Tech 2 and didn't find anywhere that it would calibrate the speedo off tire size.
Looking for the Tech 2 video, I did see a bunch of aftermarket devices that will reprogram the speedo calibration based on tire size that are a lot less expensive than a Diablo Predator. I think it will still be more than the $100 quote you already have in hand. Sorry for the bad advice on the Tech 2.