5% larger tires (effect on odometer/MPG calculations?)
 

If I am using 5% larger circumference tires wouldn't this mean instead of getting 30 mpg I would of actually traveled 31.5 miles meaning 31.5 mpg?

Sounds right to me. Taller tires should should make the odometer show fewer miles traveled than actual and any mpg calculation based on the measured miles be under reported.

yes, that is correct.
Which is why a scangauge is so wonderful.
It can be calibrated to the correct numbers.
I use my gps to get the exact distance traveled for my scangauge calibration.
As a note: I switch to metric on the gps and scangauge. It's more precise.
Then switch back to miles after setting it.

That assumes that the tires are otherwise the same. If you have a different make and/or model of tire, there will be differences in rolling resistance - and some of those differences can be large enough to offset any gain you get due to diameter.

I went from 26.7" o.d. Potenza re040 soft perfomance tires to 28" bridgestone blizzacks. The rr seems to be improved to my surprise and the lower final drive a bonus. The blizzacks are a lighter wheel tire package and my mileage has been improved greatly!!

You might be seeing better gains than you think. You gotta find out the difference in circumfrence, not o.d. I put 225/75-15's on my ranger instead of 225/70-14's and those are under 2 inches taller. Go to a tire size calculator (just google search it) and enter your stock size and the new size and it will tell you what percentage it throws off your speedometer (and therefore your odometer). In my case that less than 2 inch o.d. Was worth 7.11%, so now when I enter my mileage I multiply the odometer reading by 1.0711 and get actual miles driven

The stock odometer may not be accurate...do a gps distance run and see what your odometer shows compaired to gps distance....gps speed to speedometer is not the same as distance

I have a scanguage and I have speed ajusted properly. I calculated from my gear ratios which shows 2070 rpm at 60 mph.

Absolutely correct.

My car had 50 series tyres when bought ( 55 series is standard).
When checked against my GPS over my 52 kilometer commute, the odometer was correct and the speedo fast.
After changing to 60 series tyres, my odometer is 5.7% slow and the speedo about 1% slow.

When calculating my fuel consumption I now multiply the odometer reading by 1.057.

As it happens, I've gained about 5% FE, probably due as much to the Kumho KU25 to Pirelli P6 change as to the size change.

Imagine that!!!!
Lowering the final rpm at cruise REALLY does work!