I've started a mileage log in Google Docs. The first thing I noticed is that since the ONG station only supplies 3,000 psig gas and the TGT station supplies 3,600 psig (which is about 20% difference in capacity), my gas mileage from fill to fill will vary quite a bit. So it's the average mileage over several fills that is useful.
The only disappointment with the car so far has been that I could not drive it to Terrell, TX because there are no filling stations along the Indian Nation Turnpike...
Here's the link to the whole gas mileage tracking spreadsheet.