It seem as if all for nothing

[shonus]

So today my dad and I had to drive a total of 90 miles in my car and I felt it was the perfect opportunity to measure my aerodynamic mod improvements. Most of the trip I was driving 60 or 55 mph with a few areas 45-35 for only a few miles total, cruise control used the entire time. On the way up I had the ac on for all highway for +55mph areas and windows down through the city while on the way back the ac off and windows only cracked an inch. I re did my front undertray so it levels off with the chin spoiler, have a belly pan with only the exhaust, catalytic converter, tranny, and oil pan exposed, rear undertray sloped at 10 degrees with a little duct for the hot air from the exhaust to follow out by the muffler and pieces of coroplast to guide air from the wheels that comes inside out the rear bumper. My car is lowered for those of you whom have never seen it, the front of the chin spoiler is 5" and the sides are 4" from the ground and the skirts are around 6" from the ground. I have rear tire deflectors that I made as well. Additionally I sealed all the big gaps around my headlights and hood with weatherstrip and foam. I partially have my front grill blocked and covered the foglight holes. After doing all of this I only netted 37.55 mpg. I know I should be thankful for just that but it makes me mad all the work and money I have put into my undertray and belly pan and sealing up things and striving for aerodynamic improvements. Yet the last time I made the trip it was the same, 37.55 and I didn't have undertrays, tire deflectors, no sealing, belly panels, fog light holes were exposed, grill wasn't blocked at all, and my dad had the window open about 4 inches, nor I didn't have cruise control. Additionally that mileage was with other city driving and a few other highway trips on that tank as well.
Does anyone have any suggestions because I just saw on here another saturn owner with the same model but a 99 and got 38 mpg with no mods and bone stock and in much colder temperatures. I thought I should have gotten at least 40 mpg if not more all the time I have put into improving my aerodynamics.