This is taken from the 1929 Indian notebook. Roads where a bit different then, narrower with more hills and more dirt roads so a bit slower mean speed was the norm. With hard inflated tires on roads of today, a tooth or two less on the rear sprocket should not be a problem in most cases.
Standard factory gearing Indian Scout 37ci:
Solo Standard: 21-40
Solo Heavy Duty: 20-40
Sidecar Standard: 18-40
Standard factory gearing Indian Scout 45ci:
Solo Standard: 21-36
Solo Heavy Duty: 22-40
Sidecar Standard: 19-40
Standard factory gearing Indian Chief 61ci:
Solo Standard: 21-40
Solo Heavy Duty: 20-40
Sidecar Standard: 19-40
Sidecar Heavy Duty: 18-40
Standard factory gearing Indian Chief 74ci:
Solo Standard: 22-40
Solo Heavy Duty: 21-40
Sidecar Standard: 19-40
Sidecar Heavy Duty: 18-40
Standard factory gearing Indian Four:
Solo Standard: 17-40
Sidecar Standard: 15-40
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It is easy to make a grid chart with different numbers to get the feel of different gearing. Just divide the numbers with each other.
36/21=1,714 and 40/22=1,818 as an example.
That will show that 36-21 is about similar to 38-22 or 34-20 gearing.
40/22 is about the same as 36/19, but a small front sprocket wears more on the chain and sprockets.
I found that 19" wheels equals exact one cog less. 19" wheels with 36 teeth sprocket = 18" wheels with 35 teeth.