[brucepick]

I listen for the sound of air rushing past the car. On a no-wind day there's a distinct increase in sound between about 60-63 mph. Of course it gets gradually louder as speed increases but there's a major increase in the low '60's. Of course every car will be different but you can listen for the sound of rushing air. More sound = more turbulence = reduced FE.

I take this as an indicator of what speed to target. Wind speed affects the sound level. With a strong headwind the sound increases happen at lower road speeds, and the opposite for tailwinds. So since I have to drive the actual current conditions I now go mostly by the sound of airflow - plus of course whatever urgency I have in getting to my destination or dealing with traffic.

I commute 60 mi. each way. Partly state roads at 40-55 mph with several miles between lights, and partly open highway posted at 55-65 mph. Also some city roads at one end of the drive with frequent lights. No FE instrumentation in my '89 car.