I ordered a set of gears and a new chain for my bike. I went with the same 17 tooth front and went to a 38 tooth rear rather than the standard 42.
What a difference!!!!!!!!!!! Driving on the highway is so much nicer. I do not have a tach so I do not know the exact reduction of rpm's but I can say this. Going 90 now feels like 65 used to. Needless to say I am at the low end of 5th gear at 65 instead of the top end.
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How much the RPM's changed is a matter of arithmatic.
Assuming your 800's transmission gearing is the same as an '03 Classic, the change you made from 17/42 [2.471:1 ratio] to 17/38 [2.235:1 ratio] is a 9.5% reduction of RPM's.
Stock 90 MPH at ~5,170 RPM, w/new 17/38's at 90 MPH, ~4,680 RPM
Stock 65 MPH at ~3,725 RPM, w/new 17/38's at 65 MPH, ~3,370 RPM
A world of difference - You do lose some low-end torque, but, for me, the comfort factor more than makes up for that.
My 18/40's [2.222:1 ratio - from Scootworks] are close to your 17/38's [2.235:1 ratio].
I'm very pleased with all my mods to date, but I consider the highway sprockets to be my best mod by far.