I added the 2nd post just to cover the concept that the altered milage from larger tires will also be hampered by HP loss due to the energy required to spin the larger mass of the larger tires.

It gets more complex: At city speeds the larger tire will not challenge the engine as much, so larger tires will mostly cause you to travel further than you thought, so adjust the mpg by (tire diamter/28.5) to correct.
At freeway speed, and especially hills or any other heavy work at speed, then the tire mass will beging to work against you by increasing fuel consumption. But the mpg calculation remains as above.

Added tidbit:
Factory speedmeters are deliberately set to read ~3% to 5% higher than your actual (GPS tested) speed. This automotive detail is provided by Dr K.
I tested my trailering tires (215/75 15, or 27.7") on the Colorado trip, and the speedo was out -5%. Similar testing on my 31's (when new) showed the speedo out +3%. So doing the math, it looks like our speedometers will read exact on 30" tires, and be out +7.7% with 33's. This is compared to ACTUAL distance covered with a GPS, tested for minimum 100 mile (160km) distances, uninterupted constant speed, constant elevation, and carefully measured.

So:
If you want "REAL" MPG #'s, you need to know the speedo correction for each tire size (YOUR tire size), and remember that the factory odometer is out by -3% to -5% on the `87 A/T (and that this is an industry-wide standard).

Gentlemen, start your calculators!


edit:

Ok, Ok.

215's -5.0%
225's -4.0%
235's -3.0%
29's -1.8%
30's +0.6%
31's +3.0%
32's +4.5%
33's +7.7%