It would be helpful for the database if you could add a date field to the tank entry. As it is, I have to enter the date in which I filled the tank into the "Other notes" field. If you could associate a date with the tank entry you can see on a timeline how the persons' mpg changed over the year.

Possibly, you could also grab the actual temperatures from the persons zip code using a weather almanac database and associate it with the dates in between individual tanks of gas, i.e. average the temperatures between the two tank entries for that persons' zip code.

One climate data website is located here
http://www.cdc.noaa.gov/USclimate/