Tire size for pickup with 3.08 re

I was happy when I bought my 92 f150 pickup…untill I realized that the 3.55 re stated on the vin had been replaced with 3.08. I am working on changing those….but my truck needs new tires now. I would like to move to 16" rims with 30" tires. What is that going to do to performance and mpg? I have the 4.9l i6 in it. It seems that 16" rims have better selection for stronger, taller tires – maybe even cheaper as well.