Smaller Wheels On A Gen 5: Is There Any Consensus?

To clarify, that's not a universal statement - it also depends on the rim diameter and tyre profile. There's not a simple formula - rather there is a huge look-up table in a standards document of exactly what rim sizes every individual tyre size has to be compatible with.

You could put 225/50R19 on 6.5, although diameter would be somewhat oversized for the Prius (+4.4%). But if you tried reducing the profile to 225/45R19 to get a good diameter match, those are no longer a fit on 6.5. Even 215/45R19 are not a valid fit.

My go-to reference to check this is Rim & Tire Size Calculator. Custom Offsets - Wheel-Size.com which is looking sizes up in the official standard sizing tables.

So I would agree that in practice, you should stop at 215/50R19 for the 19" rims. 225/55R17 would be possible for the 17" rims.

 

A 225 wide tire of any profile is too wide on a 6.5 inch wheel.

 

I wouldn't want to go that wide either, and I'm not recommending anyone do it, but I'm just trying to make clear that I'm not aware of any formal rule or standard saying that, while I am aware of official standards denying that.

There are official fitment standards, and those do permit some 225 wide tyres on some 6.5 inch wheels. In the cases it's okay, 6.5" is the lower limit, but not beyond it. Because of those standards, the relevant tyre sizes will be qualified for 6.5" as much as their preferred 7.5" - tyre manufacturers are aware of the wheel ranges their tyres have to support, down to 6.5".

If you want to repeat your claim, you're going to have to clarify what you mean by "too wide".

 

Why?

For 17 inch tires, I see Michelins listed as accepting a 6 to 8 inch range in wheel width.

What is the problem a 225 width tire would provoke?