I am thinking about getting the biggest tires I can fit on my 3" truck without any rubbing

I’m pretty sure I can squeeze 33-inch tires on my truck with a 3-inch lift without any issues. If I want to go bigger to 35s, I might need to do some minor adjustments like using a heat gun to mold the fenders a bit. It shouldn’t be too difficult.

I am currently using 35-inch tires with a leveling kit that includes 5100 shocks. Everything works fine, except for the gas mileage, which has gone down.

You can fit 35-inch tires without any rubbing issues as long as you keep the tire width at 11.5 inches or less. That’s what I did.

I can confirm this. I have 12.5-inch wide rims with 305 tires, and at full lock to the left, I rub against the rear crash bar in reverse. I also had to remove the front crash bar.

As a former vehicle safety standards engineer with NHTSA, I can tell you that the front crash bars are mainly there to achieve higher ratings for overlap crashes and don’t serve a critical purpose. Only crew cab pickups have them, which suggests they’re not absolutely necessary. They do offer some benefit, but the rear crash bar is more crucial for preventing the wheel from entering the passenger compartment. It’s designed to redirect the tire away from occupants, though in real-world situations, there’s still a chance the tire might penetrate the compartment.

The offset of your wheels really matters when choosing tire size. I had 33-inch tires with a -24 offset and they rubbed a lot. I had to do a lot of trimming to make them fit.

If you keep the original wheel offset, 35-inch tires should fit without any issues. I have 12.5-inch wide wheels with a 2.5-inch lift and 35-inch tires. To make them fit, I had to remove part of the front bumper.

Even after that, the tires still rub on the back bumper and inner fender when I turn the steering wheel all the way to the left, especially in reverse or when off-roading over bumps. Aside from that, there’s no clearance problem.