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Hi John,I think Peter hit on it.
If you look at the SAE protocol for coastdown tests you notice that the aero drag portion isn't out of the picture until down around 20-mph,where rolling resistance basically dominates the road load from there on down.
Since air drag varies as the square of velocity it builds geometrically as your speed increases,and as mentioned,if you get into a headwind your 'airspeed' will climb even though your speedometer is blind to it.
And since the horsepower necessary to overcome air drag varies as the cube of velocity ( at double velocity you hit twice as much air,twice as hard,twice as often ) a headwind can really mess with you,especially on that portion of your commute when your doing the higher speed.
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