Most TT's come from the factory with 7:1 rear. That equals 17 mph.
5:1 gears will give an easy 25 mph, and if you run high rpm will give you 35.
5:1 gears are scarce and relatively expensive. But you need either 5:1 gears or an auxiliary transmission with over drive to run a TT at over 17 mph as recommended by Ford if you have 7:1 gears.
I have a set of used, but serviceable 5:1 gears for a TT. Let me know via pm if you are interested...