Here are some pictures of the metal work I had installed under my 1997 960.
I hope it will give you some ideas of things you can do to your own car for better mileage.
I gained about 2.0 mpg at 65 mph with these modifications.
One of the guys on a Volvo board was the first to install a rear cover between the differential and bumper using a plastic office chair carpet protector. He saw an instant 1.5 mpg increase on his 94 Volvo. The open area of the rear bumper works like an air brake on these cars.
First starting at the front there was this large void in front of the tires behind the bumper and spoiler. The area was enclosed to help with air flow ahead of the tires.
Next I installed a plate under the IRS rear suspension to divert air under it instead of letting it go through the structure.
Behind the plate is a box that was built to divert air from interacting with the
spare tire hump which looks like a big donut and the rear bumper.
Did you notice any fuel economy improvements after doing this?
I saw a jump of about 2.0 mpg. I have looked at adding to this undercarriage modification but the gains would be small for the amount of work and cost of materials and labor. I had a guy that restores Corvettes and builds street rods in the area do the work. Most of the custom street guys use him for metal work.
He is in his late fifties and is old school when it comes to this type of work.
The plate is stainless steel and the rear box is made of Aluminum. The boxes up front I think are made of PVC or a similar plastic.
I am looking in the next month or so of having him make me some custom snap on, snap off rear wheel opening covers for the car. Also my mechanic and I were talking last week about dropping a ring set in the rear end. That change would drop the engine speed about 100 rpms on the highway at 65 mph. Those two changes should be good for another 1 to 2.0 mpg. I want a minimum of 30 mpg in this boat anchor. Right now it is running 28.5 to 29.5 mpg at 65 mph depending on the weather. EPA has this car rated at 24 mpg highway.
Before I spent money on aero mods beyond the low-hanging fruit, I'd look at using Redline or Royal Purple motor and gear oil. Very good stuff.
A new ring set in the rear end for only a 100 rpm gain is not cost-efficient. You could get a 100 rpm change by using taller tires on the rear wheels.
As nice as the undertray work on your car is, it could stand some improvement: Get a can of Great Stuff at the hardware store, for ~$5. Use it to spray on and when dry sculpt or sand some smooth and streamlined deflectors/fairings for the tires, a la Prius. Paint them charcoal black with spray paint. In your case, it would be easiest to remove the undertrays from the car and do the sculpting work on the bench or work table, rather than crawling around under the car.
Is that low expansion foam that "great stuff"? only stuff I've used seems to keep swelling for a week.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice