Are people who own expensive sports cars all pricks? A joke has been cited in print since at least 1986:
 
Q: What’s the difference between a porcupine and a Porsche?
A: The porcupine has little pricks on the outside.
   
The sports car in the joke changed by at least 1990:
 
Q: What’s the difference between a BMW and a porcupine?
A: Porcupines have pricks on the outside.
 
         
Wikipedia: Porsche
Dr. Ing. h.c. F. Porsche AG, usually shortened to Porsche AG (German pronunciation: [ˈpɔʁʃə]), is a German automobile manufacturer specializing in high-performance sports cars, SUVs and sedans. Porsche AG is headquartered in Stuttgart, and is owned by Volkswagen AG, which is itself majority-owned by Porsche Automobil Holding SE. Porsche’s current lineup includes the 718 Boxster/Cayman, 911, Panamera, Macan and Cayenne.
 
Wikipedia: BMW
BMW (Bayerische Motoren Werke, or Bavarian Motor Works in English) is a Germany-based company which currently produces automobiles and motorcycles, and also produced aircraft engines until 1945.
 
The company was founded in 1916 and has its headquarters in Munich, Bavaria. BMW produces motor vehicles in Germany, Brazil, China, India, South Africa and the United States.
     
Google Books
The Washingtonian
Volume 22
1986
Pg. 305:
“I’m not a sports-car kind of guy,” says WWDC’s top jock, Doug “Greaseman” Tracht. “You know the difference between a porcupine and a Porsche, don’t you? The porcupine has little pricks on the outside.”
       
Google Books
Outrageously Offensive Jokes IV
By Maude Thickett
New York, NY: Pocket Books
1987
Pg. 113:
What’s the difference between a Porsche and a porcupine?
A porcupine has pricks on the outside.
       
7 January 1987, San Francisco (CA) Chronicle, “Meat-and-Potatoes Actor / ‘Police Academy’ star has no apologies” by Peter Stack, pg. 48:
“And I (Steve Guttenberg—ed.) said, `But Sid, I like the Toyota. I mean I feel right about it somehow. I hear Gene Hackman drives one. And other guys in the business.’ Well the agent says I should be nice to myself and at least go look before I tie down the Toyota deal. We look at BMWs and I can’t stand the thought of spending 35 grand on a dumb car. Then we look at Porsches and I get snapshots in my brain of the kind of guys who drive Porsches, and I don’t want to be that kind of guy, if you know what I mean about the old porcupine joke - the difference between a porcupine and Porsche is that with Porsches the pricks are on the inside.”
 
Google Books
The Chase:
A Novel
By Robert Stein
Iowa City, IA: Press of the University of Iowa
1988
Pg. 34:
Her lawyer story was about the difference between a porcupine and a Porsche with two attorneys inside.
   
Google Books
The Romantic
By Aram Saroyan
New York, NY: McGraw-Hill
1988
Pg. 162:
What’s the difference between a Porcupine and a Porsche? The pricks are on the outside of the Porcupine.
 
Google Books
Carnival Desires
By Mark Lindquist
New York, NY: Atlantic Monthly Press
1990
Pg. 80:
“What’s the difference between BMWs and porcupines,” Jayne asks as the valet pulls up in Bick’s car. “With porcupines,” she smiles, “the pricks are on the outside.”
   
Google Books
Lawyers & Other Reptiles
By Jess M. Brallier
Chicago, IL: Contemporary Books
1992
Pg. 16:
The difference between a porcupine and a Porsche with two lawyers in it is that the porcupine’s pricks are on the outside.
 
Google Books
1001 Internet Jokes:
Get the Jokes You’ve Been Missing
By D. M. Schwab
Lincoln, NE: iUnverse.com, Inc.
2001
Pg. 100:
What’s the difference between a porcupine and a BMW?
Answer: Porcupines have pricks on the outside.
     
Google Books
Don’t Even Think About Telling this Joke to a Lawyer
By Harry Bergen
New York, NY: The Berkley Publishing Group
2007
Pg. ?:
Q. : What’s the difference between a porcupine and a pair of lawyers in a BMW?
A. : The porcupine has its pricks on the outside.
 
Twitter
Bruce Bechhold‏
@bruceb
Her Royal Highness Duchess Andrea: Do you know the difference between a porcupine and a BMW?  With a porucpine, the PRICK is on the outside.
5:14 PM - 20 Oct 2007
   
19 January 2008, Townsville (Queensland) Bulletin, “Junk (F) Email Bush goes to Heaven” by Andy Toulson, pg. 33:
Q: What’s the difference between a porcupine and a BMW?
A: A porcupine has the pricks on the outside.
 
Twitter
Men’s Humor‏
@MensHumor
The difference between a porcupine and BMW is that the porcupine has the pricks on the outside.
6:30 AM - 4 Sep 2011
 
Twitter     
Men’s Humor‏
@MensHumor
A porcupine is kinda like a BMW… except a porcupine has its pricks on the outside.
11:00 AM - 2 Jan 2012
 
Reddit—Jokes
Whats the difference between a BMW and a porcupine?
submitted December 21, 2013 by crazyhomie34
BMWs have the pricks on the inside!
COMMENTS
Acidic_Jew
Joke critique: I think it’s funnier if you use the punchline, “porcupines have pricks on the outside,” as the listener/reader gets a revelation of shifting viewpoint and uncover “pricks on the inside” on their own.
 
Twitter
Todd Kirkpatrick‏
@TKirkpatrickYLL
Replying to @thisBshane @Stany_72
it’s an old lawyer jk: what’s the diff btw a porcupine and a lawyer in a bmw? W the porc, the pricks on the outside.
12:23 PM - 21 Jul 2016
 
Reddit—Jokes
What is the difference between a porcupine and a BMW?
submitted November 11, 2017 by DeadYen
A porcupine has pricks on the outside.
 
Reddit—Jokes
What is the difference between a BMW and a porcupine?
submitted January 9, 2018 by dirtydaversfg
Porcupines have their pricks on the outside!!!