There is a factor called 'curve resistance' which is eliminated in road tests by keeping to a perfectly-straight road course.
Curve resistance occurs when tire scrub keeps a car along the path of the curve,when centripetal force would rather sling it off the road in a straight line.
Highway engineering texts must cover this aspect of automobile behavior in road construction for safety reasons.
I've yet to see a uniform equation for which to accurately calculate it,complicated by so many rubber compounds available in tires.
The radius of the curve,any super-elevation of the roadway,the velocity,mass of the car,tire compound (as mentioned),inflation pressure,actual road surface,detritus on the road surface.etc. must be included in the equation.
At Ontario Motor Speedway,California,Alex Tremulis measured a 15-mpg penalty while driving through the tracks curves at 55-mph aboard the 100-mpg Subaru X-100 Gyronaut.
It is this curve resistance that your Ultra-gage is reacting to,not aerodynamics.
What you're doing may help otherwise though.
