The High/Low, 4:1, 48mm gearboxes are what you would want for the Tiger 1. They have the lowest ratio and will improve the slow speed maneuverability a lot. You can find them here: http://www.taigentanks.com/taigen-tank-parts/tag14010s
There is also a video on the page as well going over the new upgrades to the gearboxes if you would like more info.
You're lucky you have hills, I have just sand here! Happy tanking!