I have the same question. The T165/90D18 donut is the same diameter (within 0.1" on a tire calculator, which could be rounding error) as the P245/55/R19 main tires. But the full size is 3 inches wider.

The muffler is right beside and just a few few inches from the unshielded donut, so I don't see any more problem with the wider full size tire, other than a small reduction in ground clearance. However it won't stick down any more than the factory 4-way tow connector at back and little more than the drive motor just a bit forward (and I'd rather bottom first on the tire than the motor).

Anyone see a problem I don't with mounting a full size spare? The 2019 HiHy manual even shows an alternate tire rotation that includes rotating the spare with the two mains on one side.

 

Hmm...now I wonder if the previous owner of my 2012 Base put a full-size spare. Do they come with a donut & steel wheel by default? In either case, I've had it for a long time underneath and used it recently as I broke the valve stem (had the aluminum cap that welded shut to the stem). No issues as far as I'm concerned. The pressure was perfect too which I was shocked.

 

Anybody know which way a full-size spare is stored, air valve up or down? It's kind of a PIA to have to lower the donut to check the air pressure.

 

Why do you have to lower it to check the pressure? Just get on your back and check from the bottom, plenty of clearance.

 

I would recommend not doing anything that moves the spare closer to the exhaust without at least adding some sort of heat shield. It is likely though that a full size tire won't fit in the factory location. I totally understand why you want a full size tire for a spare. If I were in your shoes I would buy one of the very nice, padded bag that completely covers the full size spare and deal with having it behind the rear seats. You would get the added bonus of thew spare lasting significantly longer with it being in a climate controlled space. with no water and much lower moisture.

 

I asked toyota about that.
They suggested not modifying the vehicle.
I know they sell a very expensive cover for the underside of the full size spare, 1000 CAD or thereabouts for 2 pieces of plastic.
I had a screw go into a tire on a recent road trip at dusk ,continuing another 3 hours to destination on a donut was not a pleasant experience.
In some countries those spacesavers are banned.