[user removed]

The lighter the car the lower the sectional density, a term used in ballistics to explain how heavier (longer in bullets of the same diameter) projectiles retain velocity for longer ranges.

Basically you have less mass, with which to store inertia, and with less inertia your aero drag will slow the car down faster. Confirm this by adding passenger weight too see if coasting distances increase.

The cool thing is lowering your aero will lengthen coasting distances more that it would with a heavier car, so your aero is more important. Also less mass means the same bsfc will provide you with quicker acceleration, evidenced by your 44 mpg climbing a hill.

regards
mech