MPG With Lift/Tires? Also, How to Calulate Difference

The first four days with my DCSB Sport 4x4 AT I was seeing 22.6-21 MPG. I put on a 3'' lift with 275/70R17 Cooper AT3's and I am seeing 17.5 MPG consistently. Those with similar lift and tires, what are you seeing for MPG now?

Also, since the tires are larger I am actually going further than what my speedo says. So I should be adding mileage to the speedo correct? The circumference percentage difference and add it to miles traveled?

The picture attached shows the difference between stock and current tire. Looks like the new tire is 5.2% larger therefore I would add 5.2% distance to what my ODO clocks?

Thanks!

 

BUMP

 

For the most part thats correct. Whatever the percentage difference is for the size is what you would add to your mileage.

So for mine they are 3.3% larger, so I add 3.3% to the mileage when I'm calculating mpgs.