A 195/65 profile is almost identical to the stock size MC tire profile (maybe a couple mm taller). Your highway RPM should be pretty similar. The 175/80 profile Austone was about 14mm taller profile, which translated into about a 300rpm drop on the highway and a slight mpg boost.
195/75 tires are available (lots of choices, it's the size VW camper vans use apparently), but these are considerably taller, another 5-6mm more profile than the (formerly) popular Austone. Some people report rubbing issues with the Austone particularly with 2 up riding or using aftermarket/shorter shocks, so I suspect a 195/75 would only be worse in that regard and too tall to really recommend. Personally I have no rub marks from my Austone, but I ride solo almost exclusively, oem shocks, on the firmest setting.
I've searched around quite a bit and am really shocked I haven't been able to find ANYTHING in the 175/80 size being sold in the US. That's the size the London Black Cabs take, look at a Euro based website and there's dozens of choices in that size that are cheap as hell (the Austone that sold for $200 here goes for about $45 equivalent in the UK) but it's unobtanium apparently this side of the pond.
This is the closest I've found, a 175/75-16. Same narrower 175 width as the Austone, but a smidge shorter. Still a bit taller than the stock MC tire, it'll probably reduce RPM by more like 150 instead of 300 at freeway speed. But the narrower width is what is of more interest to me.
https://www.ebay.com/itm/RADAR-Argonite-RV-4-LT175-75R16-101-99R-8-Ply-Quantity-of-1/192955905834
