[aerohead]

I completely taped all the panel gaps on my truck for the wind tunnel and the difference in drag could not be measured,so I don't anticipate any savings for you there.
Simple pizza-pan wheel covers would help.
Wheel skirts would help.
The belly panels would help.
I tried expensive,full-synthetic transaxle lube (Red Line Racing Oil) in my Honda,and couldn't measure any savings.
On the open road,at highway speed,the greatest saving would come from boat-tailing,or a box-cavity device.The box-cavity would be the easier to fabricate.The boat-tail would have the highest performance.Perhaps a 15% mileage improvement by itself.