I went to acuraparts website and it reports that my car wheels are 18" in size:

Acura Parts @ AcuraAutomotiveParts.org - Genuine Acura OEM Parts from Acura Carland

At the same time posts like
https://acurazine.com/forums/tires-w...-guide-656174/

make me think that my wheels are 17".

I have the following tires installed:

Michelin Pilot MXM4 Performance Touring All Season
P225/50R17

So, I am wondering if these tires will fit 18" or are guaranteed to be on 17" wheels.

I am looking to buy the same size wheels for winter tires and having trouble finding a cheaper alternative that matches OEM wheels with +55mm offset.

 

Your wheels are 17"

As for a winter set of tires, there are many options, and you could especially find a good deal on a set of winter tires on stock 2nd or 3rd gen TL wheels or even accord wheels. Check the black market and craigslist.

 

If you have P225/50R17 tires then you have 17" wheels. 17 inch tires cannot fit on 18 inch wheels. All 4cyl TSX came with 17" wheels. Only the V6 came with 18" wheels.

For winter, you'll want either the same size tires or a narrower tire that still has the same overall outside diameter.

 

215/55/17 is a good alternative size. Slightly narrower and taller than stock tires, but MUCH better pricing than 225/50/17.

You missed out on some black friday deals. I got $160 cash back at discount tire on a set of already discounted Bridgestone WS80's.

No snow in sight, though. :/

They were doing $320 cash back if you bought a wheel and tire set. The second set of wheels makes it extremely convenient. But if you're not installing them yourself anyway, you may consider just mounting them on your existing stock wheels.