Tractor-trailers are complex beasts; every aspect of the unit’s performance depends on dozens of factors within not only the truck but the trailer also. A truck’s purpose is to tow a trailer, which is why the trailer makes up most of the unit’s size and mass. So, you don’t need to ask what the tractor’s turning radius is, since the unit’s overall turning radius and diameter usually depends more upon the trailer than the tractor. Either way, all you need is to apply a bit of simple trigonometry to calculate the truck’s steering and Ackerman angle.
Measure your truck’s wheelbase in inches from the center of the rear axle hub to the center of the front axle hub. If you have tandem drive axles, as most tractor trailers do, then measure from the center of the empty space between the tires on the two rear axles.