Ok just to make it clear as mud I have a a 2014 f150 with a 3.55 gear ratio 5.0 engine I'm currently running 275/75r18s and I just recently put on 1.5 wheel spacers I want to put 35in nitto trail grapplers on but I'm very much on the fence about weather I'm asking for something to blow up down the road prematurely. l may take my truck offroading 2-3 times a year and I don't tow anything and I hardly bag my truck never over 4000rpm. I'm not very educated about this stuff so I figured this would be the best place to ask thanks!

 

I would go with 4.10s.

 

Assuming you started with the factory size at 265/65/18 and you've moved to the larger 275/75/18 tire, you have increased the axle ratio not quite 8%. Going to a measured 35" tire means another 3/4" of tire. The truck had as an option, the 3.31 axle ratio. You'll be just about at the same as having the standard tires and the 3.31 axle. The 3.31 ratio was considered part of the Fuel Economy package with the 5.0. It neuters the 5.0 powerwise pretty good but still very drivable. It would not be the axle ratio of choice for any kind of heavy offroading. The suggested 4.10s would be far better but at the expense of fuel mileage.