I can say that I have never been able to achieve 70 MPG unless my speed was below 55
MPH. But that doesn't mean that it can't be done. Two things that could have contributed to your 73 MPG was a tail wind and/or a decrease in elevation.
I have done a lot of testing at different speeds and have developed a speed vs MPG curve for MY car. Under optimal conditions, warm temps, no air conditioning, no significant winds:
- 70MPH = 50MPG
- 65MPH = 55MPG
- 60MPH = 60MPG
- 55MPH = 65MPG
- 50MPH = 70MPG
- 45MPH = 75MPG
- 40MPH = 80MPG
- 35MPH = 85MPG
The most important thing is to only consider round trips. That way you can rule out a change in elevation. I also carry an altimeter on board which reports elevation in increments of 5 ft. Watching the altimeter is an eye opening experience. When I thought I was climbing, I was actually flat or falling, and vice versa. I now rely on the altimeter instead of my instincts.