You can use a quarter as a tread-depth indicator to determine when your tires need replacing. Hold the coin upside down in each of a tire's long grooves.
If you can see all of George Washington's head, the tread depth has worn to an eighth of an inch and it's time to think about a new set.
Many places sell tires, including local car dealers, independent tire stores, regional chains, "club" stores such as Costco and Sam's, tire-company stores like Goodyear and Firestone, and online outlets such as Tire Rack and Discount Tire Direct. Sales and rebates are common, and prices can vary dramatically. Any of those outlets might offer the lowest price on a given day. The key is to shop around.
In late August, for example, we priced our top-rated all-season tire, the Michelin LTX M/S2, at several retail outlets. They included tire retailers and discount chains in California, Colorado, Connecticut and Illinois, as well as several online vendors. The price per tire ranged from $212 to $271 at tire stores and $206 to $245 online. Buying online usually adds shipping costs, normally $20 to $25 per tire, but occasionally it's free.
The X factor could be the cost for mounting, balancing, and related costs. They can add $12 to $30 per tire. At regular tire stores, our installed, out-the-door prices for a set of four tires ranged from $944 to $1,225. If we had bought the same four tires online, we would have paid $924 to $995, including shipping. Then, if we assume an average of about $20 per tire for installation at a local shop, our totals would have been $1,004 to $1,075.
Some tire stores also push extras like road-hazard insurance, although it probably isn't worth the cost unless you have to replace tires often because of road-induced damage. In some areas, you might also have to pay a disposal fee for your old tires.
Compare the installed, out-the-door price when shopping for tires. If you can get a good discount from an online site, call around to local shops to find the best price for installing them. Websites can often ship directly to a local tire store. And if a local store promises to meet or beat other legitimate price quotes, that could be the best route of all.