Food for thought:
We're in a January thaw for most of this week. Temps for daily highs are forecast to be above freezing for the entire week, with the average daily high somewhere around 6C / 43F.
That'll raise dad's numbers above where they would normally be for winter driving.
ALSO...
FYI: from
fueleconomy.gov's Shared MPG Estimates for Toyota Corolla ....
2008 (automatic) - avg mpg / range [# vehicles] = 31.1 / 27 - 34 [7]
2007 (automatic) - avg mpg / range [# vehicles] = 31.8 / 23 - 41 [18]
2006 (automatic) - avg mpg / range [# vehicles] = 33.2 / 23 - 44 [32]
2005 (automatic) - avg mpg / range [# vehicles] = 31.1 / 24 - 41 [32]