Getting better. So far 3 tanks down since purchase:

Trip Calculated Summary

18.8 N/A Iffy gas from dealer, and having a bit too much fun

20.3 20.56 Lots of idling w/AC, getting used to the throttle, very little hwy

22.4 23.77!! Trip seriously underestimated, quite a bit of hwy, driving more "normal" now

I have a GT-Reserve, and keep track of my mileage in a spreadsheet.

On average, I get 22.8 MPG using 87 octane, and 24.4 MPG using 93 (based on 3 tanks of each). Given that I have switched up from one tank to the next, I think the "undiluted difference" might be greater.

Regarding my spreadsheet, I calculate each tank's MPG manually, and also enter what the computer says. I then take those two figures and calculated how many cups of gas more/less I should have bought for the two MPG figures to match.

I found that while the two MPG figures do not always match on a tank-by-tank basis, they equal each other over time (after about 2,000 miles and 6 tanks of gas, they are within .08 mpg of each other). My biggest spread on an individual tank has been 1.6 MPG.

On an individual fillup basis, I once calculated that I would have had to purchase an extra 3.93 cups of gas in order for the two MPG figures to match. Then the next tank, I would have had to purchase 3.91 cups

*less* for them to match.

The back-to-back swings have been as high as 11.52 cups more then 12.72 cups less.

The middle variance was 6.00 cups more/6.13 cups less.

So the individual tank MPG calc difference all depends on how full that tank was to start, and how full I got it at that fill up. Not all gas station lots are level. And I may have not been as patient each fill up.