You were driving through a snowstorm the other day. A row of FWD minivans and pickup trucks with bald tires crawled along in the right lane, many with their four-way flashers blinking. Then a semi truck scooted by in the left lane, making pretty good time. You wondered, How many drive wheels does an 18-wheeler have?

A standard American semi-truck tractor routes engine power to two rear axles. Each axle almost always has dual wheels. That means eight driven tires.

The truth about 18-wheeler trucks

Mechanic Tim Charlet took to MyMechanic to explain that a classic semi-trailer truck is also called an 18-wheeler because it has 18 tires in total. The front two tires steer and handle much of the braking. The next two axles are the drive axles, with four tires each.

Some specialty rigs — such as logging trucks — use a driven front axle, but those are rare, and you’ll rarely see them on the highway. Other semi trucks have a single driven rear axle. This simplifies the powertrain and prevents wear and tear.

The first ten wheels belong to the tractor. It tows a semi trailer, which usually rides on two rear axles. Those axles also carry four tires each. Add those eight to the tractor’s ten, and you get 18 wheels.

Of course, there are exceptions. Larger multi-trailer “trains” can have far more trailer axles. Some trucks have lift axles (also called tag axles) that lower via air suspension to support unusually heavy loads. But no semi trucks have driven trailer wheels — there’s simply no way to route power back to them.

Semi trucks in the snow

Yes, semi trucks have eight drive wheels. But that isn’t the secret to their winter traction. What matters is that much of the weight from the cab and trailer presses down on those eight tires, keeping them firmly planted.

When truckers need even more grip, they may wrap chains around some or all of their drive tires. Chains limit top speed, but they make it extremely difficult for the tires to spin out. You can learn more about how a semi truck works in the video embedded below: