If you look at it, both the Calmini and the TT are 4.24:1 - I'm guessing that TT just copied Calmini's design, which has been in production for years. I have the Calmini set. Yes, you can still do high-speed wheeling in low range, as long as you consider 20 MPH (in 5th gear) to be high speed! Those babies are LOW! I love 'em!
Take some precautions, when you're "clearancing" the case - you have to scrape or file some of the aluminum off. When I did it, I covered the hole the gear goes into, with duct tape, to keep filings out, then washed the filings out with solvent.