[argonaut]

I believe stock tire size that would have come on the pickup is a 235/75R15, which is about 29" in diameter. Now you want to step up to a 35" tire

So 3.08 original gear size / 29" original tire diam * 35" new tire diam = 3.72 theoretical new gear size

Then take into account those 35" tires are much wider, and heavier, and are probably a less efficient tread pattern. This takes more power to spin and overcome friction. So we'd bump up from the 3.73 we calculated, to a 4.11.

This will probably get you back to similar performance as what you originally had.

So now consider how you will use your pickup. Will you be hauling much payload, or towing much? If so you might consider MTCK's recommendation and go up more to a 4.56 gear ratio to put the motor into a stronger part of the powerband at cruising speeds.