Does anyone have a chart for rpm /speed for a 2 speed rear axle? I was sure I saw one somewhere but I have been through the 48 spec manual and the owners manual and haven't found one. With 8:20 x 20 tires
I just created one when I needed it. I don't fully understand the tire sizing, so I just looked up the revolutions per mile for my tires and went from there. I made one for a NP540GL that had both the low gear ration options, and the higher gear ratio options. I later found out that I had the lower ratios (6.4 and 8.7), and made another chart for an SM420 and the low option (deemed "LO" as opposed to "HO" on the spreadsheet).
If you found out your tire revolutions per mile and had a google account so I could share the google doc with you, then you could punch in your own numbers and play around with it.
SM420 Chart
NP540GO Chart 
