40 cogs at the rear sprocket and 19 at front is for sidecar use in a fairly hilly area or perhaps solo with a 37ci engine. It gives a ratio of 2.10. (I think 40-20 or 36-18, at 2.0 ratio might be orig. gearing for a 37ci solo.)

Orig. solo gearing for a 45ci is 36-21 that gives a ratio of 1.71. To have  about same ratio with a 40 rear sprocket you need a 23 cog that would give 1.74 ratio. I don't know if a 23 would fit the chain cover at front, but a 40-22 would give 1.82 ratio and that's just slightly undergeared, about the same as 36-20 at 1.8 ratio. I am running 36-21 with 19" wheels and that is still undergeared for my taste, on flat roads. When it's time to change chain, I'd try 36-22 at 1.636 for a higher cruise speed, and get used to use the second gear more. A 35-21 would be something in between at 1.666 ratio. If you only want to change one sprocket, you might try 32-19 that gives 1.684, but a front sprocket change with that small rear sprocket gives larger jumps in ratios...still 32-18 would be useable at 1.777.

And Stuart, very welcome to the group! Hope to hear more from you!

I found in a Indian notebook from -29 the factory sprocket gearing for the various models. The 37ci,45,61 and 74 has all the same gear ratios inside the gearbox. Here's the sprocket counts:

Indian Scout 37, Solo standard 21-40. Solo Heavy duty 20-40. Sidecar standard 18-40.

Indian Scout 45, Solo standard 21-36. Solo Heavy duty 22-40. Sidecar standard 19-40.

Indian Chief 61, Solo standard 21-40. Solo Heavy duty 20-40. Sidecar standard 19-40. Sidecar heavy duty 18-40.

Indian Chief 74, Solo standard 22-40. Solo Heavy duty 21-40. Sidecar standard 20-40. Sidecar heavy duty 19-40.

Indian 4, Solo standard 17-40. Sidecar standard 15-40.

Indian Prince, Motor sprocket 15, Clutch spr. 38, Countershaft spr. 38, Rear wheel spr. 43.