But just because you can do this on your own car, does that mean you can really do it to a rental? Do you realize that if something goes wrong either during the tire change or related to the tires you have on the car, you'll be liable for a whole bunch of money and none of your insurance will cover it (because you've violated the terms of the rental by "making changes" to the car)?

And removing a storing tires from the rental car with a third company? Are you serious?????????

And what if the car "naturally" breaks down in the middle of the trip and you have to take it to a local Avis place and exchange it for another car? (This has happened to many us at one time or another.) Then they'll spot the "wrong" tires on the car, and then what? (You can't get back to the original places now to swap the tires back before you exchange the car, that's too far.) You're "up a creek without a paddle", as the saying goes.

(Did you realize that if you have trouble with the car, standard rental company policy is to exchange the car rather than fix it on the spot?)

I thus think it's a really risky plan to change tires on a rental.

"Tampering" with a rental (to the rental company, that's what changing the tires on the car you rented is) could well cost you even more than you paid for those airline tickets if they catch you!

And won't that disrupts your itenerary (and life) a little more???

There are other way to handle this. For example, get a second rental after you get to Canada, leave the original rental parked somewhere cheap, then pick it up when you're ready to return to the US. Or take a train from New York to Montreal, rent there, then return there, and take a train back.

But changing tires yourself on the rental sounds like such a horrible idea, that one I think you should take off the table. It shouldn't be your last choice, it should be a choice at all!