I am test driving a 2017 F150 XLT Crew Cab RWD with a 3.5L engine. It is not the ecoboost.
While driving at highway speeds >55 mph I have to fight with the steering wheel either pulling to the left or the right. It starts off with the right tire then it's anyone guess. Has anyone had this problem and what causes it?
The truck has 27,xxx miles.

 

What do the tires look like?

 

The truck has tree good Michellen's and a cheaper replacement on the left front.

 

Well was it balanced? Seems the one bad tire is the issue, or there is a replacement tire there because somebody hit something and bent up suspension components. Hard to diagnose a front-end wobble on the internet though.

 

I'd be immediately suspicious of the tires, especially that "cheaper replacement".

 

Sounds like someone ruined a tire and it had to be replaced. It could also be a bad rim or something more serious related to the suspension. Low mileage coupled with steering issues at highway speeds on a used vehicle raises a big red flag.

 

Might be as simple as moving the new tire to the rear.

The tread pattern and wear on a tire can also cause it to pull. Different brands and even the same brand, with different wear patterns, do not have the same rolling characteristic. For instance, replacing one tire in a set may cause a pull. If we place the new tire on the front, with an older tire, the vehicle may not drive straight. Replacing tires in pairs is always best, and the new tires should go on the rear.

http://www.agcoauto.com/content/news/p2_articleid/307

 

Steer clear of this one. Someone probably curbed it hard enough to blow a tire!

 

Two things causing it, since you said it pulls and not vibrates, the issue is tread mismatch and EPAS. More than likely the 3 are original with 27K and the other is newer with more tread, and since not a matching tire, will cause alignment issues in that the two front tires will fight each other for the direction they want to go.

Because of this fight, EPAS is detecting drift and compensating for it, which when there is no drift, results in a pull. Overcorrect the pull and it will now pull in the other direction, and this back and forth will go on until you stop, or hit a bumpy road.

I would have the tires rotated and test drive it again, chances are it will no longer pull.