Ok, I'm about to swap my stock, 235/75/17(Hankook crap) to 275/70/18(Gdyr ATS) off a Lariat.
How can I adjust the speedo so that it's still accurate? The dealer said these computers cannot be changed for different tire sizes. Is this correct or are they idiots?

 

They're idiots. I think the dealers can do it. Superlift makes a Truspeed calibrator that will take care of it. And if you by a tuner, most will hanble that.

 

They can change it, but they may not have that particular size in their programming. If done by them, you may have to go with the closest they have. As mlawdawg states, there are other options.

 

Use your head...that's what I did on my old car.
Speedo difference will be 7.379% too slow. 60MPH stock = 55.8ishMPH new.