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The fact that you can easily feel a difference while drafting means that your aero is awful. The more aero a vehicle is the more extreme draft will be required to detect a perceptible difference.
I think you were initially correct when you mentioned both aero and momentum. I think if you took a look at your CdA/mass ratio it will be the defining factor it how well/how far you can EOC at highway speeds. Higher weight and lower CdA will coast further; higher CdA and lower weight will not coast as far.
Motorcycles have horrific Cd, but low A. Both are rather rider-dependent, as is the running weight. Knowing little about bikes, I'll go with some assumptions that between rider and bike you're looking at 1000 lbs, Cd of .5 and half the area of a car. Your car let's say has a Cd of .35 and a weight of 3000 lbs. Your CdA/mass ratio of the car would be 1.17x*10^-4 (x being your total area of the car) while your motorcycle would be 2.5x*10^-4. Since your bike has the higher CdA/mass ratio it will decelerate more quickly than the car (higher drag per momentum). Even if the bike had only 1/4 the area of the car you are still marginally higher on CdA/mass ratio, which will get even worse with a lighter bike or fewer BigMacs. It's not purely scientific and won't help you calculate your actual deceleration, but it's a good method of determining the trend.
You are correct though that at low speeds the vehicle with less rolling resistance (the bike) should coast better.
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