That's an option. But I'd like to know how much duty would cost me too see if it's worth the risk. I don't want to get flagged at the border. Then get hassled every time I cross.
If you're going to buy the tires in the U.S. then having them mounted down there is the best bet as long as you don't have to bring back your old tires. I've done this several times with no issues at the border, They would have a hard time proving you bought them there to begin with. Who's to say you didn't buy new tires before you entered the U.S.. It's not what you know, it's what you can prove.
In answer to your original question, You would not be charged any duty as long as the tires were made in the U.S., but the catch is vey few tires are made in the U.S., I think the only ones are Goodyear and some Cooper models.
Here's a link to a tire company in Canada that has good pricing and real cheap if not free shipping.
2011 Silverado 3500HD SRW LML CCSB LTZ. Line-X over the Rail, Rockers and Side Steps, S&B Intake w/Donaldson Powercore G2 Filter & Scoop, Kennedy Twins/W Filter, Edge Insight CTS, Crown Performance SS Brake Lines, PPE Rear Diff Cover, Bushwacker Fender Flares, Cognito Leveling Kit with UCA's & Steering Braces, Rare Parts Tie Rods, Bilstein 5100's, KC Light Bar, Rigid Dually D2's, LT295/70R/18 Terra Grapplers, Allison Deep Trans Pan, Transynd, Suncoast Filter Lock, Retro Solutions 5000K HID's in my Fog Lamps, Snugtop Canopy plus a few other Goodies.