Trying to figure out MPG based on larger tire size.... Does this seem right????

I just bought my 04 AV z71 a few weeks back. It has a 6" lift and 35s. I knew my speedo was off due to the fact I'm running 35s. I've looked online and tire size calculators that take into account the factory tire size 265/70/R17 vs. the current 325/70/R17 tire size. It says it will throw my mileage and mph off by 10.5%. I just want to make sure I'm figuring the gas mileage as close to correct as possible. So I went to the gas station, checked my trip meter and it read 252 miles. I add an additional 10.5% to come to 278 miles. I then divided that by the gallons I filled with 16.7.  Which means I'm getting 16.7 mpg. That just seems really good for having 6" of lift on 35s. Am I overlooking something?