So...it's bad, but not necessarily as bad as it seems.
Yes, the taller tire raises the body increasing frontal area and aerodynamic drag, and a wider tire both increases drag from road friction AND increases the frontal area.
But, it's not all bad. A taller tire also increases the tire's circumference, which makes the engine's gearing taller, slowing your acceleration and giving you lower engine RPMs at the same speed which is a good thing for MPG.
However, you never really see that benefit unless you do careful math and testing, because now both your speedometer and odometer are wrong. What was 69MPH is maybe now 73MPH, and what was 1 mile is now 1.07 miles. That means your truck is doing more real work by going faster and further than your MPG display is telling you.
I had fun testing this one out after putting the big 285s on my truck. Doing before and after testing, using GPS to make sure I was driving at the same real speed, and to correct the odometer distance, I found that after the upgrade my real MPG was about 10% higher than the computer. So 15MPG is really 16.5MPG.
Food for thought...and as always, YMMV
