I never compared mine with hand calculated numbers, has anyone taken the effort to do so over a long term run?

You only went up an inch in diameter so the reading probably would have dropped a bit, not much.

reads 18 and good ol' math tells me closer to 16. these are numbers from when im hard on the skinny.

Mine stock was always 2-3 mpg better than what was actually calculated. Now with superchips it's dropped to 1-2 mpg better than calculated.

Yep: Superchips recaled and gettin 1 mpg actual mileage less than dash display

same here...my calculated numbers are about 1-2 mpg less than what my jeep tells me

Are we talking about resetting the ODO and the MPG meter when we fill up the tank and compare numbers when we fill up again?

Yep, starting from zero each time.

My computer average is off 4/10 of a mile per gallon on the high side.

Same here, about 1-2 mpg less when calculated. Computer milage is just wishful thinking.