Good gas mileage?

As we were moving the last of our things to our new home--packing my car and his truck with more stuff than they have ever carried for a distance of about 550 miles--I was thinking 'our gas mileage is going to be just awful with all this weight.' When we've made this trip in the past, the mileage on my car was generally about 34 mpg, and I didn't expect anything better on this trip. Surprise! 37.6 mpg! Now, if I could only figure out how THAT happened...