Yeah - I remembered last night that you're running a snowmobile clutch, so it's probably in overdrive...
If you use:
ClutchRatio = (RPM x TireHeight) / (MPH x 336.135 x RearGear)
The 46mph comes out to a 0.62:1 clutch ratio
If you add that back in to:
MPH = (RPM x TireHeight) / (RearGear x ClutchRatio x 336.135)
an 8.1 gear would give you 79.5 mph.
Of course the snowmobile clutch reacts to torque, so if you gear up the rear end too much, it may not open up to .6 anymore, which is going to slow it down. When you're dealing with that, it is going to take some trial and error...
Is Heidi going to let you go 80mph on that thing???