[lincpimp]

May have answered my own question, but have a look and see if I am anywhere near correct...

Pinion 13t
Spur 50t

Diff gear 40t
Counter gear 44t

And the spur and counter gear have 15t gears attached to them.

So here is the speed calc with a 55t motor on 3s lipo with Moab xl tires.

Differential Ratio: 2.66
Transmission Ratio: 2.9333
Other Ratio: 1
Spur Tooth Count: 50
Pinion Tooth Count: 13
Total Voltage: 11.1
Motor KV: 1150
Tire Diameter (inches): 7.25
Tire Ballooning (inches): 0
Spur/Pinion Ratio: 3.85 : 1
Total Ratio: 30.00992 : 1
Tire Circumference (inches): 22.78 inches (578.52 mm)
Total Motor Speed: 12765 RPM
Vehicle Speed: 9.17 mph (14.74 km/h)
Effective KV Value: 1150

So Chip Cross seems to think a pair of 45t motors will be a bit better than the 55t motors, but with 7t pinions instead of the stock 13t pinions... That would drop the speed to about 5 mph, which is a bit slow for my uses.

Maybe a 10t pinion, that would be around 7mph.