“The bicycle is a curious vehicle. Its passenger is its engine” is a quotation from American cyclist John Howard that has been printed on many images. Howard wrote in The Cyclist’s Companion (1987):
 
“The bicycle is a curious vehicle whose passenger is also its engine.”
 
 
Wikipedia: John Howard (cyclist)
John Howard (born August 16, 1947 in Springfield, Missouri) is an Olympic cyclist from the United States, who set a land speed record of 152.2 miles per hour (245 km/h) while motor-pacing on a pedal bicycle on July 20, 1985 on Utah’s Bonneville Salt Flats. This record was beaten in 1995 by Fred Rompelberg.
 
Google Books
The Cyclist’s Companion
By John Howard with Albert C Gross and Christian Paul
Lexington, MA: Stephen Greene Press
1987  
Pg. 157:
The bicycle is a curious vehicle whose passenger is also its engine. In over 30 years of cycling, I’ve been inundated by information about the nutritional and physiological factors that determine whether and how comfortably that engine will keep going. I hope I have successfully sifted through that bicycle lore to separate the facts from the myths, because in this chapter, I present what I know about the engine itself, the fuel that you put into it, and the accidents and mechanical breakdowns ...
 
Google Groups: alt.mountain-bike
Quote of the Donkey
willjune
5/15/98
“The Bicycle is a Curious Vehicle, it’s Passenger is its Engine”
  John Howard
 
Google Books
Go Fly a Bike!:
The Ultimate Book about Bicycle Fun, Freedom & Science
By Bill Haduch and Chris Murphy
New York, NY: Dutton Children’s Books
2004
Pg. 33:
“The bicycle is a curious vehicle. Its passenger is its engine.” —John Howard, who once rode his bike 152 miles per hour behind a windscreen on the Bonneville Salt Flats
 
Google Books
The Bicycling Training Journal:
A Daily Dose of Motivation, Training Tips, and Wisdom for Every Kind of CYclist—from Fitness Riders to Competitive Racers
By Editors of Bicycling Magazine
Rodale, Inc.
2004
Pg. 38:
“The bicycle is a curious vehicle. Its passenger is its engine.” — American road racer John Howard
 
Google Books
Reconsidering the Bicycle:
An Anthropological Perspective on a New (Old) Thing
By Luis Antonio Vivanco
New York, NY: Routledge
2013
Pg. 11:
But people are also actively involved in powering and directing bicycles, a dynamic reflected in john Howard’s famous observation, “The bicycle is a curious vehicle. Its passenger is its engine.”
 
Google Books
Biteback Dictionary of Humorous Sporting Quotations
By Fred Metcalf
London, UK: Biteback Publishing Ltd
2013
Pg. ?:
The bicycle is a curious vehicle. Its passenger is its engine.
John Howard, American pro-cyclist
 
Google Books
Words to Ride by:
Thoughts on Bicycling
By Michael Carabetta
San Francisco, CA: Chornicle Books
2017
Pg. 8:
The Bicycle is a curious vehicle.
Its passenger is its engine.
JOHN HOWARD