The OEM 275/65/18 Continental tires on my F250 are well worn at 33k miles. I have been debating up going with a taller and/or wider tire to fill the wheel wells a bit more. Much of the info I find o...
Larger tires will change the effective gearing by an amount proportional to percentage of the diameter. That may be a small amount. However, larger tires weigh more. Your OEM tires might weigh 50 pounds each while the proposed 34" tire weighs 65 mounds. You will likely feel that in terms of more fuel used, increased brake wear, and slower acceleration.
I experienced exactly this with my prior tow vehicle. It sure looked sweet, but going back down to stock size really woke the truck up.