This tank
Has had both "3:1" and "4:1" gearboxes in it, but to achieve scale speed I now run the "3:1" gearboxes. With a decent board, like a TK22 or an IBU, slow speed is no problem and scale top speed is pretty much right on.
The only "5:1" boxes you can get right now have steel gears and won't be very good in a bulldog. They're good in a P3, but that's about it.