When I'm driving down the highway, my car wants to pull to one side. How come?

A steady steering pull or "lead" to one side may have any of a number of causes. The most likely cause is wheel misalignment. This may be due to rear wheel toe or axle misalignment, front wheel camber misalignment, too much cross camber or caster alignment (more than a degree of difference side to side), or someone having "aligned" the front wheels without the steering wheel being properly centered beforehand. In any event, it will probably be necessary to have the alignment checked to diagnose and correct the problem -- unless one of the following is causing the pull:

An underinflated front tire on one side. Check tire pressures and make sure they are the same side-to-side (no more than a couple of pounds of difference).

Mismatched tires. Tires of different size, aspect ratio or even tread pattern on one side can create enough of a difference in rolling resistance to cause a pull.

A weak or sagging spring. Measure and compare ride height on both sides of your vehicle (measure at the fender openings). If one side is an inch or more lower than the other side, chances are you have a spring that needs to be shimmed or replaced.

A dragging brake. This can be caused by a frozen or sticking disc brake caliper that doesn't allow the pads to kick back out from the rotor or weak or broken return springs in a drum brake that don't pull the shoes back from the drum. Another possibility here might be a packing brake that isn't fully releasing on one side.

An uneven load. If you, your significant other or a passenger is causing your vehicle to lean to one side, it can cause the steering to lead in that direction. Don't laugh, a few hundred extra pounds can make a big difference in a small vehicle -- especially if the weight isn't evenly distributed side-to-side. If you can't do anything about the extra weight, it is often possible to compensate by having the wheels realigned with a "simulated" load positioned in the vehicle. Of course, then your vehicle may lead in the opposite direction if the extra weight is removed.

Excessive road crown. Roads are usually sloped (crowned) from the center towards the sides for drainage. If you spend a lot of time driving on highly crowned roads and find the constant lead to the outside shoulder annoying, you can have the wheels realigned to compensate for the excessive crown. Adding or subtracting camber from one wheel or the other to create a difference in the cross camber alignment of your front wheels can counteract this kind of problem.