I found this interesting equation about how to get your truck running like it was designed to after you change tire/wheel sizes.

First determine your wheel diameter of your new and old tires.
If you don't feel like measuring or are unsure check out this site.

http://www.net-comber.com/tirecalc.html

My example:
Old tire size 235/75-16 = 29.88 inches
New tire size 275/55-20 = 31.91
Gear Ratio = 3.42

Step 1 [Old Tire Size]/[New Tire Size]*[Current Gear Ratio] = [New Gear Ratio]
29.88/31.91*3.42 = 3.20

So switching to 20's actually makes my 3.42 act like 3.20 (this sucks!!)

Step 2 [New Tire Size]/[Old Tire Size]*[Gear Ratio] = [New Gear Ratio]
31.91/29.88*3.42 = 3.65

So to get my truck back to factory optimized parameters I need to increase my gears to at leat 3.65 (3.73) to get to back to decent gas mileage.

I hope everyone enjoys this......

 

Yup, that's it! Effective gear ratio. That's really what matters. I constanly see people decide or debate which gear ratio is better without any regard to tire size. Because a 4.10 is too much in your buddy's f-body doesn't mean its too much in your truck.

 

Exactly, by using those equations if I use 4.10 in my truck with my wheels it will almost be like having stock 3.73's with stock tires.

 

How much of a decrease in mileage did you see?

 

My truck came from the dealer with the 20's on it so I don't know. But my father bought the same truck a week later with the regular wheels on it and he gets like 18-19 mpg. I am getting around 15. But he drives likes his age....