Found this online while researching a 1990s Mustang that was heavily modded for top speed. Are they wrong about the Airdam?
TheMustangNews.com - How-To: Top 5 Mustang Mods That Suck Gas
1. Big Wings & Air Dams
Yep, they look awesome. Aftermarket body augmentations are the best and most eye popping way to suck more gas. Wings are the best. The larger the wing, the more drag is created. The same goes for the huge front fascia with the low to the ground air splitter. Sure it helps stability on the road course, but it does increase your frontal area by leaps and bounds. It takes more power to push all that surface area through the air, and thus more gas. Potential Efficiency Decrease: 1-3mpg