“Gas, Grass or Ass—Nobody Rides Free!” was a popular bumper sticker, cited in print since at least May 1978. The message means that, to pay for a ride, the driver will accept gas money, marijuana or sex. The bumper sticker was widely popular and is still in circulation, although “ass” frequently comes first (“Ass, Gas or Grass—Nobody rides for free!”).
   
   
Amazon.com
Ass, Gas or Grass - No One Rides For Free - Sticker / Decal
by Unknown
 
Biker T-shirts
GAS GRASS OR ASS NO ONE RIDES FOR FREE
 
Urban Dictionary
ass gas or grass
The most acceptable forms of payment for the favor of being given a ride to any destination. Roughly translates to: money, weed, or sex (oral sex is also acceptable). Gifts should be given promptly and without hesitation.
(...)
by BES the best Jul 3, 2006
 
Google Books
May 1978, Baseball Digest, pg. 91 ad:
Gas—Grass or Ass
Nobody Rides Free!
(Jumbo bumper stickers—ed.)
 
Google Books
Abbey’s Road
By Edward Abbey
New York, NY: Dutton
1979
Pg. 159:
The bumper sticker in front of me read: Ass, Gas, or Grass — Nobody Rides for Free.
 
Google Books
Naked Pictures of Famous People
By Jon Stewart
New York, NY: Perennial
2001, ©1998
Pg. 48:
The new rules are: Ass, gas or grass, nobody rides for free; and Be cool.
 
Google Books
It’s Too Late to Leave Early
By William Walling
College Station, TX: Virtualbookworm.com Publishing, Inc.
2002
Pg. 331:
“There’s an old bumper sticker that says, ‘Ass, gas, or grass. Nobody rides free’? It’s my numero uno live-by policy.”