2016 Prius Four Touring....Whoah! Those aren't alloys!

As far as the 15" vs. 17" fuel economy in the US... they don't have to report the figures if they sell less than 33% of the cars in that configuration. So, that's how you do it here - sell less than 33% of the cars in that configuration. The EPA figures are for 15" wheels (I forget if they're for Two, or for Three/Four). Based on this post, it looks like EPA mileage for the Tourings would be roughly 49 city, 46 highway, 48 combined, if Toyota were required to report it.

That's what Toyota Germany answered? What does your local government say? They're the authority that would fine you for having the wrong tire size, after all.

The steering ratio is actually slightly different per Toyota UK specs - 13.4:1 for 15" wheels, 13.7:1 for 17" wheels, IIRC. And, turning circle is wider for the 17" wheel setup... but the 15s should work fine on a 17" car, it'd be the other way around that I'd be worried about (and then, not really).

 

That's why I always end up a bit diagonal in parking spots.

 

The government says what it is written on the papers. If I have only 215/45R17 wheels allowed, that is all I am allowed to put on, unless I buy new tires and try to get them approved myself, or somebody (the dealer or the tire shop) does it for me and gets my papers modified.
I am now wondering if when I got my Gen3 I had only one tire type and then when I got my winter tires, my dealer had it approved also for the smaller winter tires....

You can say "in Europe", not just Germany, rules are very strict about any modifications that pertain the exterior, and sometimes the interior, of the car. Also if you change something like a light bulb in a car under warranty, if something happens and it is due to the change (even indirectly due to), you are on your own and pay out of your own pocket. A lot of the changes I see guys report here in PC would void the warranty in no time. Including, likely, do it yourself oil changes within the warranty coverage time period.
I wanted to change on Gen3 the park lights and put some which were of a different point of white. I could surely buy them on Amazon, but there was a note that they were not approved by the Traffic Authority. This meant, sure put them on, but if they stop you and get fined because they are not allowed, your problem....

 

That honestly sounds like a terrible place to be a car owner. Well at least one that somewhat enjoys individualism and modifying ones car for pleasure?

This is all of the EU you say?

 

There are many regulations that are harmonized among all the EU countries. They are more guidelines than rules, and like all others, they are decided centrally and then need to be approved and put into practice by each single country and converted to law there.
But because we have already 50+ years of economic and goods exchanges, there have been rules which have been very similar for years already, to allow for example that if I move to another EU country I don't have to have a new driver's license. I just need to exchange it to a new one local one in case I stay more than 6 months and I am registered in the new country. This was already the case before the introduction of the Euro and Schengen.
So also for cars, some "rules" or "guidelines" are widely accepted in all EU countries - also because if I allow in one country to be able to do all sorts of changes without any verification, the car cannot be e.g. sold in another country, driven in another country, can cause accidents more easily in another country with a completely different climate (e.g. Italy vs Finland) and so on.
Essentially all rules are there to make sure easy interchange and safety, and reduce costs for manufacturers'. One car model can be sold in tens of countries in one basic version, instead of 20 different variants.

 

I wish they'd do that here - particularly with SUVs, the owners change the ride height, then the lights which blind you when they drive behind you. They'll put huge "Bull Bars" on which surely can't comply with regulations, but nobody checks, nobody seems to care. Exhausts are changed - I've got 3 cars up the road with noisy exhausts which would have been put off the road 20 yrs ago - worst is a Golf TDI DSG with an exhaust which sounds like ... loud flatulence.

Police only care about driving EXACTLY at the speed limit.