"My other car is a [very expensive car]” is a saying that has been printed on many bumper stickers and license plate frames and put on cheap old cars. “My Other Car Is a Porsche” is a bumper sticker that was cited in print in January 1979.
“My other car is a bicycle” has been cited in print since at least 1983 and is popular with cycling enthusiasts. “My other car is a Zamboni” (the ice resurfacing machine) has been a popular license plate holder for hockey enthusiasts since at least 1988.
12 January 1979, Omaha (NE) World-Herald, “The famous rowboat caper” by L. M. Boyd, pg. 8, col. 1:
This week’s bumper sticker: “My Other Car Is a Porsche.”
24 March 1979, Indiana (PA) Evening Gazette, Family Leisure, pg. 4:
But she is still driving around in a little Honda although she has splurged and has a frame on her license plate that reads: “My Other Car Is A Rolls-Royce.”
4 April 1979, The Register (Santa Ana, CA), “Unique Auto Market,” pg. C8, col. 1:
The license plate frame read “My Other Car Is a Porsche,” a small deceit.
Google News Archive
20 September 1981, Modesto (CA) Bee, pg. C-1, col. 4:
Then invest at an auto supply store in a license frame that proclaims “my other car is a Rolls Royce.”
January 1983, Orange Coast Magazine (CA), pg. 173, col. 3:
Then, also, there are all those inane little license plate holders which have given us such notable sayings as “I’d Rather Be Golfing,” or “Hit Me, I Need The Money” and who could ever forget “My Other Car Is A Mercedes.”
Google News Archive
21 September 1983, Lodi (CA) News-Sentinel, “Cycling fun for commuters” by Judy Larson, pg. 3, col. 1:
“The license plate frame on my car says ‘my other car is a bicycle’ and it’s literally true,” he said.
“MY OTHER VEHICLE IS A HORSE” bumper sticker. $2. Horselaugh. P.O. Box 1861, Evanston IL 60204
28 June 1985, Sacramento (CA) Bee, “She was just too careful,” pg. S16:
You know those immodest “My Other Car Is A Porsche” and “My Other Car Is A Cadillac” license plate frames, usually seen on old clunkers?