Highway Cruising

As most of you know who have been following this blog, my commute is pretty short. I have been taking the streets and the route has some 45, 35 and 25 mph stretches with a number of hills, stop signs and traffic lights. Now I have taken the car on the freeway a few times for test purposes but just for an exit.
Lately, when I get in the car for the drive home at the end of the workday, the SOC meter will show 95% SOC. This can be attributed to the fully charged SOC showing over 100% as the Zivan will charge the string above what Optima said for 100%. In any case, last week I decided to take the drive home on the freeway. It was a bit tense as the SOC reads zero under load and I was pulling about 300 amps at 60 mph, but it was a fun ride. After sitting for an hour the SOC showed 45% - not bad.
Over the weekend I picked up a stock Miata "under cover" - this the plastic piece that goes under the engine between the nose and the subframe. My car didn't come with one and I wanted it to keep water off the motor when driving through puddles and for aerodynamics. I also picked up a sheet of aluminum for blocking off the air intake.
This week I've been commuting on the freeway just like the big boys. I'm now pulling around 250 amps at 60 mph (not sure if this has to do with a lighter touch on the pedal or the under cover) and getting about the same SOC displays as I was taking the streets.

My explanation for this is that the distance is about the same but there are no hills or stops to waste energy on.