My parents have a 2003 Matrix. They average about 30 mpg with it doing mostly highway driving. There's one little detail about it that's been driving me nuts. The front bumper.
There's a huge gap!
It's so big, I could stand in it if I wanted to.
That's got to make all sorts of stupid votexes and lets air that goes in the lower grill dodge the radiator. So, this past weekend, I made filled the gap. Bolting a plate to the lower edge of the bumper was an easy decision. The rear mounting point required at least five minutes of thought.
The factory under trays start at the bottom of the radiator and go down a bit and back a bit. They are mounted very securely. You can see them here infront of the oil pan and transmission pan.
I bolted the back edge of my new panel to the factory trays. The panel is made of an aluminum realistate sign I got for free from a scrap yard, and some left over chunks of street sign from my previous project, also obtained from a scrap yard.
The panel ends about an inch further back than the original tray, but still give enough room to get at the oil filter. That means I wont have to remove it for oil changes.
The whole thing was painted black before final mounting. I don't know if the fuel milage was improved at all, but at least the ground can't be seen through the front grill anymore.