The vast majority of new cars sold in the U.S. come fitted with all-season tires. As the name implies, these tires are designed to meet the basic needs of drivers in various climates across the country.
Because of this they must balance the need for sufficient dry and wet grip against the desire for low rolling resistance in the interest of high fuel economy ratings. Most are also asked to be quiet, comfortable and long-lasting. And for drivers who live where winters are harsh, they need to provide a passable level of grip in cold weather and snow.
But Jack-of-All-Trades thinking seldom produces the best results in any single category. With this in mind we set out to find out how much all-season tires might be giving up to dedicated winter tires in the face of frigid conditions. Is seasonal tire-swapping the way to go? How big is the difference, anyway?
A Few Words About Tire Types
When most people think of winter tires, they think of aggressively treaded tires with metal studs in them. Such tires exist, but they're designed for extreme conditions. For our test, we used non-studded winter tires. Not only are they allowed on all roads (studded tires are often restricted), but a tire's tread pattern is not the only factor that determines how well it works on snow and ice. The chemical formulation of the rubber and how it behaves at low temperatures plays an equally vital role.
Snow tires utilize rubber compounds formulated to produce grip at low temperatures and feature a tread pattern that's oriented to bite into snow. A network of razor-thin cuts called "sipes" further subdivides the tread blocks into numerous gripping edges. The result is a squishier tread surface than you'll see on other tires.
Summer tires are better known as "high-performance" or "three-season" tires. Their tread comprises large solid tread blocks with high lateral stiffness and no real sipes. Instead they have wide circumferential and swept-back grooves made to expel and even sequester water, and their sticky warm-weather rubber formulation gets rock hard at lower temperatures.
All-season tires contain elements of each. Their tread pattern consists of medium-size tread blocks that provide some lateral stiffness along with a suggestion of biting edges. Sipes are present, but not in great numbers. And they employ an intermediate rubber compound that stays compliant throughout a wider temperature range.
We shipped a collection of carefully selected tires to a winter proving ground operated by Automotive Enviro Testing (AET) in northern Minnesota. But our intent was not to survey the mind-bogglingly huge tire aftermarket and determine the single best winter tire. Instead we chose a single brand and compared products within its lineup in order to see how much winter tires really help.
The choice of tire brand followed the choice of vehicle: the 2009 Honda Civic Si. We selected this car because it's offered with a choice of summer or all-season original equipment tires in identical sizes. Today's 2015 Honda Civic Si continues to be unique in this regard because optional summer performance tires are typically wider than their all-season counterparts.
On our 2009 example, both tires were Michelins in size 215/45R17, the all-season Pilot HXMXM4 and the summer Pilot Exalto PE2. Michelin also makes a winter tire in the same size, the Primacy Alpin PA3. Because of this, the 2009 Honda Civic Si allowed us to isolate the effects of tire type while holding the size and manufacturer variables constant.
That said, many advocate narrower winter tires on a performance model like the Civic Si, the idea being that skinnier tires bite into the snow more readily. So we ordered a second set of otherwise identical Primacy Alpin PA3 winter tires from Tirerack.com in size 205/55R16 to see if their modest 10mm (0.4-inch) reduction in tread width made a measureable difference.
Why include summer tires? Because people travel and relocate. The sticky high-performance rubber that was ideal in Florida will turn dangerous if you drive to the northern U.S. for the holidays or accept a job offer someplace where they have a real winter. Our aim was to quantify this issue with comparable test results.
AET maintains a huge variety of test courses that automakers use to perform numerous winter-related tests that go beyond tire performance. We focused on one of them: the groomed-snow Vehicle Dynamics Area (VDA). This expansive surface allowed us to conduct our usual straight-line performance tests on snow.
The snow VDA is so vast there's not much to hit. And the uniform groomed surface made it possible for us to shift our start and stop zones around for consistent conditions each time. All test passes were made with traction control and ABS up and running. And each data point is the average of the middle three of five test runs.
We did make one other fundamental change to our routine: The acceleration and braking tests were capped at 40 mph instead of 60 mph. It's a more relevant test speed in wintry conditions and it's something of an industry standard for this sort of work.
Snow Acceleration Results
The typical dry-weather acceleration run is a wide-open test of a car's maximum acceleration where the engine is the star. But in winter conditions such limit tests become a real-world measure of how well the tires hook up when you're simply trying to get moving after the stoplight turns green.
On its factory all-season tires, the Civic took 14.5 seconds to reach the modest speed of 40 mph. The same-size winter tires improved the picture dramatically to 11.7 seconds and the skinnier winter tires brought it down to 11 seconds flat, an overall improvement of 24 percent relative to the standard all-season rubber.
And the summer performance tires? It's almost too sad to mention. It took them an excruciating 41.7 seconds to coax the Civic up to 40 mph.
Snow Braking Results
Maybe you own an all-wheel-drive vehicle that gets going more readily than a front-drive machine like the Civic. You'll still have to slow down, and in sloppy conditions the unexpected panic stop is far more commonplace. Winter braking performance is crucially important to all vehicles, regardless of the number of wheels driving them up to speed.
Here the all-season rubber stopped the Civic from 40 mph in 184 feet. The same-size Michelin snow tires did the deed in 156 feet, a two-car-length advantage that could be the difference between stopping safely and rear-ending someone. The skinnier version of these winter tires brought that down to 147 feet, some 37 feet and 20 percent shorter than the all-season originals.
As for the summer tires, they skated their way to a stop in a miserable 351 feet, almost double the distance required by the all-season rubber. You simply do not want to get caught out on high-performance summer tires in snow or frigid weather.
Winter tires made a big difference to the measured acceleration and braking performance of our Civic Si test vehicle on a test-grade snow surface. Dedicated winter tires are the easiest performance and safety improvement we can imagine from a simple bolt-on change that you buy and quite possibly even install by yourself in your garage.
What's more, the slightly narrower version of the same tire accounted for a further measurable improvement that may be worth pursuing if you're looking for maximum cold-weather performance.
All of the above is from the assumed starting point of all-season tires. Anyone who plans a holiday drive north or a ski vacation into snow-covered mountains on summer performance tires needs to rethink their travel plans. And those who relocate from warmer climes to a place where winter is no joke need to budget money for a set of dedicated winter tires if their car doesn't at least have all-season tires.
Even if it does, the extra confidence and margin of error provided by winter tires will be a big help to those who lack cold-weather driving experience. For that matter, even the most experienced winter drivers need the best tires they can get.