I'm not sure why everyone is so afraid of paying duty. For one thing on any smaller items <$300 they generally don't bother with duty, many times you'll get assessed the HST... Big deal, you pay HST every single time you buy something here.
As for duty yea it's ridiculous on some things, like liquor and tobacco, sure, maybe that's where the fear comes from but on other stuff it's not that bad. I was looking at appliances and a fridge made outside of NAFTA is 8%, add the 12% HST and you're 20% extra, so if you happen to be saving 40% you're still ahead. Made in Mexico or the US? No duty at all, just the tax.
I don't know the duty on tires but if they're made inside of NAFTA it should be 0%.
I haven't bought tires across the line yet, my next set will be I think. Tirerack advertises shipping to Canada with all taxes, duty, brokerage and shipping paid up front. I just ran thru a set of 4 Goodyear Integrity in 225/60/16: They're $85 each and $187 to ship and then $85 in HST and brokerage and provincial tire disposal fees. $613 total to my door. Or about $137 per tire plus tax and disposal fee if you compare to local prices. It showed no duty so those tires must be made in the US or Mexico probably. Keep in mind you have to add mount and balance at probably $80/set.
You can likely save more if you ship them to a depot across the line and import them yourself. Plus that's without shopping around. For my truck tires Discount Tire Direct keeps coming up cheaper than TireRack when you factor in the shipping. Then there's that 1010 tires and some smaller ones.