Fun! I still remember the first time I cracked 70 on a secondary highway run in the Firefly: howling tailwind. I knew it, but it didn't make it any less satisfying!
I remember going east on I-64 and getting 62 MPG in the Fiesta. On the return trip I was fighting to stay over 40MPG. A 30 MPH west wind was the culprit. It's nice when it's a helping wind that dies before you return but usually works the other way.