For both my 2014 Ram and my 2009 F150 the speedometer was low with the stock tires. With the 2" taller tires I put on to replace them, the speedometer was the same percentage over as it was under with the stock tires. Going from 33.5" tires to 35.5" tires on the Ram put the speedometer over 3mph at 75 mph: hardly enough to even think about.
At any rate, it sounds like a lot of members are comparing upgraded tires to the tires that came on their truck. I'd compare them to the largest tire available on that model year truck from the factory and go from there. For example, the current F250 can be ordered with 34.5" tires stock. It's not reasonable to say that a 35" tire is going to cause issues because you are going up from the 32" tire that came on your base model. It won't. It's a half inch taller than a tire offered from Ford. Will it be a little less peppy? Sure. Will it be within what Ford designed? Pretty much.
The real issue comes from going considerably wider than stock. On my F150 I went from stock 275/55r20 to 275/65r20. You could hardly tell the difference.
On my Ram from 285/65r20 to 295/65r20, likewise. It's when people want to go from a 9.5-10" wide tire to a 12.5-13" wide tire that you get mileage and power problems due to increased wind drag and significantly higher weight.
What's always worked for me is this: stay as close to stock width as possible, go up 10 in aspect ratio, and there is rubbing only in extreme situations and little mileage and power penalty. My last 4 full size trucks worked exactly that way.
