How much would my MPG improve with a tune up?

I am wondering how much a tune up will improve my gas mileage. I have a 1999 GMC Sierra 1500 Ext. Cab with a 5.3 V8. It hasn't been tuned up in a long time and im right now getting about 12 miles to the gallon and from what I hear a V8 can get up to about 16 city.