Dark Future is more detailed than Gaslands and plays slower than Gaslands. Dark Future has many options for weaponry, equipment, vehicles, and roleplaying not found in Gaslands.
Because both systems use 1/64 scale miniatures, modified die-cast cars for Gaslands can be used in Dark Future and vice-versa. If you can find any of the out of print Dark Future weapons and cars, the Dark Future miniatures have excellent aesthetics for Gaslands.
My friend Francis Greenaway wrote Highwayman, a set of Dark Future roleplaying rules that uses
d20 Modern and the Horizon: Redline. Highwayman is available on the Dark Future Web site Francis created over 10 years ago.
Future Highways: The Online Magazine for Highway Warriors of the Dark Future
http://futurehighways.roll2dice.com
Future Highways - Highwayman: The Dark Future Roleplaying Game (Supplement for Horizon: Redline)
http://futurehighways.roll2dice.com/redline/index.html
Fantasy Flight Games published Horizon: Redline for
d20 in the early 2000s. The book is out of print.
I have seen other gamers use the Atomic Highway
RPG for Dark Future roleplaying. Atomic Highway is available as a no-cost download from DriveThruRPG.
If you like car combat games, you might be interested in my Web site listed in my signature file. It has a few hyperlinks and resources for car combat tabletop games. I have recently updated one page that might be interesting to you.
SWAT
HQ - The Megaton List of Car Combat Tabletop Games
http://www.seanet.com/~owenmp/links/car-combat-tabletop.html
