I'm shopping for replacement tires for my OEM 275-55-20 Pirellis (2010 FX4) and want to bump it up a notch to 285-55.
Is there anything that might be adversely affected ie. transmission, computer, TPS, etc?

 

It shouldn't. I have the same year and model truck and replaced my tires with 275/60-20s and it didn't have an adverse effect other than throwing speedometer and mileage gauge off a bit. A dealer and many engine programmers can adjust your computer to compensate for that. The difference in od (Outside Diameter) of the new vs OEM tires is what determines how much change there will be. Every brand tire is slightly different od even with the same "Size" tire so go a site like Tire Rack and look up the od of the exact brand, model and size tire that you're looking at and you can calculate how much it will affect your gauges. IF it's it a big difference it could also affect your true MPG since your engine will be operating under a different load and at a different RPM but I don't think it will make any significant difference in your case.

 

Joe, can you post a side and rear pic of the truck with those 275-60-20.

 

I'll try to do that tomorrow but it depends on the weather. It's been raining almost every day. These are only about 4/10" larger od than Pirellis so IMO it looks stock.