Feeding the
aero/rr web tool my car's stats, and varying just the ambient temperature, I get the following:
Temp: ... 15C/59F ... 25C/77F ... mpg change (from lower temp)
45 MPH ... 74.48 mpg ... 76.28 ... 2.36%
50 MPH ... 63.9 mpg ... 65.54 ... 2.57%
55 MPH ... 55.22 mpg ... 56.71 ... 2.63%
Avg difference: 2.52%