Per Tire Size Calculator: The 215/55/R17 are closer in diameter to oem 195/60/R17 than the oem195/50/R19's are.
With the OEM Dunlop Enasave tires, the speedometer calibration is 100% accurate at all speeds on my Gen 4 Prius Prime, every speed (except when approaching zero) being indicated exactly 2 mph higher than the actual speed.
Sounds like this is a case of the specific tire size not being used enough in the US, so it simply isn't stocked here. The gen5 tire sizes are vexing for owners here, but they are more common in Japan. Owners there don't have the same issue with the sizes. Regulations specify the error limit between actual speed and displayed speed. The OEM sizes are close enough that adjusting the speedometer isn't needed to stay in that range. As for using wider tires, Toyota choose 195 because thinner tires have better aerodynamics. Wider tires may improve performance, but they will reduce efficiency. If I had a gen5, I'd be kicking around the idea of trying the 175 wide tires made for the i3.
I am mostly hoping to get a slightly higher sidewall than stock, but without going much wider than stock. But l never thought of going narrower. Good idea. I wonder if 185mm tires will fit stock LE/SE rims well? But, now , as l look, 185/65R17 tires (which would be a great replacement, size-wise) aren't readily available either ...
Handling much improved. I drive a lot of windy mountain passes here in Colorado. Insignificant loss in fuel economy.
Yeah owners manual pdf (you MUST have downloaded by now?), tire specs are in there, and enter into online tire comparison calculator. IIRC with 5th gen, the stock 17’s are about 1/2” smaller diameter than 19”. With the 15” and 17” on gen 3 and 4, it was other way around, and 1/4” diff at most.
