They largely determine how well a car brakes, corners, accelerates, how comfortable and quiet its ride is and how much gas it consumes. Yet people who pay hundreds or thousands of dollars for marginal improvements like rear disc brakes, ABS, stability control, even all-wheel-drive, a "sport-tuned suspension" or higher fuel economy choose the cheapest available round black things when the time comes to replace tires. Few people enjoy buying tires and frequently it comes as a large unexpected expense, but tires may last you 30,000 - 60,000 miles and cost $350-550 installed. In that same time, you'll spend $3,000-$6,000 in gasoline, plus your insurance, depreciation and other maintenance.
The bad news is that there are good reasons to pick Good tires and not just Cheap tires, and I'll try to explain some of them here. That means you may end up spending more money after reading this. The good news is that there are lots of very good tires in the lower half of the pricing chart, and new tire designs are coming out and coming down in price that offer good performance at reasonable prices. My intention is just to let you make a more informed personal choice that will be more satisfying in the long-run.
It's very difficult to get fair side-by-side tire comparisons. You always end up comparing brand-new tires to worn-out tires, or different sizes of the same model, or the same tires but on two different cars, or getting opinions from different people, etc. Nobody ever gives you two Hyundai Elantras with identically worn tires to try out side-by-side. We have our own tire reviews here at the club (link), so at least the tires are all pretty new, the car is the same and most of the tire sizes are the same, even if the people are different.
Do you need all four? Maybe you haven't checked your alignment in a while, or haven't been rotating the tires like you should've, but for whatever reason, one or two tires are worn out while the others are still usable. Keep in mind new tires won't fix your alignment, so you'll ruin good tires as long as that problem isn't fixed. Secondly, mixing tires is not-optimal at best. Putting different tire models or differently worn tires on left and right sides is especially to be avoided. This gives you a nasty surprise because the effect isn't evident until you need 100% of the traction in an emergency situation when you already have your hands full! Always use matching tires at least from side-to-side. Mixing different tires on the front and rear is not as bad but still far from optimal, but the question often arises: I only need two tires, for now (/to pass inspection) do I put them on the front or rear? Ok, here:
We all try to drive carefully, but that doesn't mean we should be unprepared for emergencies - that's why we have ABS, airbags, etc. Stuff happens, stuff makes you slam on the brakes in a panic in the rain. So here's your choice: With worn-out tires on the front, your car won't stop any better (than before you bought 2 new tires). It'll skid forward longer or take longer to stop even with ABS. Also, it won't steer very sharply either, but it'll stay pointed forward. With worn-out tires on the rear your car will spin around and you'll probably lose control if you were going fast. So what sounds more appealing, the inevitability of skidding head-on into your doom, or the wild, unpredictable spin with the surprise ending? I'd say, stop being cheap and just buy 4.
One specification many people don't look at, and which is misleading, is the speed rating. The original tires for the Elantra are rated "H" or safe up to 130mph, where S or T-rated tires are still deemed "safe," to over 100mph. That just means the tires won't overheat and explode from the sheer speed alone but says nothing about the handling capabilities of the tires at high speed. Although you never drive your Elantra at 130mph, the difference between H and cheaper S or T-rated tires can already be felt at 70 or 80mph. Low-speed rated tires can make the car feel dangerously unstable or 'squirrely,' at speeds far below their 'spontaneous-blowout' speed rating. Even though you'll never reach 130mph, staying well within the capabilities of the tire is a good idea for safety and driving comfort and might save you from having to go back and return a set of tires.
Another specification is the load rating that consumers should pay attention to in making their selection of a tire. A low load rating means that the tire is capable of handling a specific weight (car, passengers and cargo). This is different than the speed rating.
P195/60R15 87S - The load index (87) is the tire size's assigned numerical value used to compare relative load carrying capabilities. In the case of our example the 87 identifies the tires ability to carry approximately 1,201 pounds.
A tire can overheat and possibly fail (shred, blow-out) if the weight is too high, even on a slow speed trip.
Most cars on the road use All-Season tires. These are a compromise to provide acceptable traction year-round from very hot summer days to sub-freezing light snow in winter but by the same token, they are never the best tire for any particular weather. (Hence the joke "No-Season tires") There is no one tire that is optimized for all conditions: The very same features that aid traction in the snow hurt dry-road performance and viceversa. Additionally, tires' performance is temperature-sensitive. You may remember a high-school or college science demonstration where a rubber band is dipped in liquid nitrogen. Frozen to such a low temperature rubber is brittle and shatters on impact. We'll be dead before our tires shatter, but as they approach this glass-transition temperature, they start to become less flexible, they contour less to the surface roughness of the road, and lose grip more easily. Conversely, on hot summer days and at high speed, tires may become too soft and slippery and wear out faster. Winter, Summer and All-Season tires have different critical temperatures that makes them best suited to cold or hot conditions, or a compromise in between. For example, winter tires can have 33% more grip in the snow than all-season tires (TireRack) ; 33% is the difference between an Elantra's and a Porsche's braking performance.
The choice of tires depends on your local climate and driving style. However, if your goal is to maximize your car's capabilities and you have to live in more than one season, you should really consider keeping more than one set of tires. There is a significant up-front cost (including a second set of wheels to avoid re-mounting/balancing charges twice a year) but your tires will last longer, you'll always have the best ones for the season, and you'll get some of it back when you sell the wheels/car. This can be a side-effect of buying more stylish or sporty wheels - you keep the original wheels as a "Winter," set and get to choose your style and preference of wheel: flashy, classy, sporty, functionally lightweight, etc. The wheel size, offset and weight will also change the feel of the car... a topic for an entirely separate Wiki article!
A term that's not usually stated or discussed as much is Rolling-Resistance. It's a pretty self-explanatory term and the summary is that there is a 1-2mpg fuel economy difference to be seen among various tire models. However, many cars come equipped with these energy-efficient tires from the factory to help the car's official EPA fuel economy rating, the poster child being the Toyota Prius. So owners often see a slight drop in fuel economy after replacing those with less-efficient models. The difference probably isn't as drastic for the Elantra though, so if you're concerned with fuel economy, optimizing your driving style and checking tire pressures often is probably just as good. There is even opposition to Low-Rolling-Resistance tires from safety groups concerned they compromise grip and safety.
You can find good deals on tires online. Sites like The Tire Rack, Discount Tires and 1010 Tires have prices that are usually better than your local tire shop, even with the shipping cost. Plus they offer dozens of models and brands that fit each car, let you compare prices and customer ratings side-by-side and read hundreds of customer reviews at home, with no sales pressure. Tire Rack even has scientific side-by-side comparisons of new tires mounted on the same car in terms of braking distances, lap times, skidpad grip, road noise perception, ride harshness, etc. The local stores (I've found) only carry Super-Cheap, Cheap, Sorta-OK and Super-Expensive tires, and expect you to decide on the spot based on the name and price only! If you're thinking that four tires might not fit in your mailbox, you can have the tires shipped to the shop of your choice and set up an appointment to have them installed. Alternatively, having the UPS guy cart four tires over to your front door will tell your neighbors that you're definitely a car nut. So I highly recommend shopping for tires online even if you end up buying locally.
Whether you're OK with a long-lasting or inexpensive T-rated All-season touring tire on your commuter or you think it's worth it to get a dedicated set of Z-rated summer performance tires and a second set of snow tires, or just move to Florida, it's up to you. I hope this gave you some appreciation about the importance of tires and leaves you a little more informed or motivated to learn than before.