The incredibly dangerous stunt was caught on camera by a Spanish driver who remarked: "This is what guiris do. 100kms and look how he is travelling."
Guiris is the slang word often used in Spain to describe northern European tourists.
"What madness," he adds.
It is not easy to make out the figures in the grainy footage until the convertible car enters a tunnel when streetlights reveal what looks like a man in his shorts sprawled on his back across the rear of the car.
The man taking the video pans down to show his own speedometer as he keeps up with the pranksters – and records 99km per hour (61mph)
The footage uploaded onto Youtube by the user karrasko82 in the early hours of Friday morning is being investigated by Malaga's Civil Guard traffic unit, according to a report in Malaga newspaper Diario Sur.
The stretch of road where the footage was filmed appears to be the newly-built tunnel between Marbella and Puerto Banús.
Tourism chiefs have warned against dangerous crazes such as balconing, a craze where tourists, usually under the influence of alcohol, attempt to jump from balconies to the swimming pool below.
In recent years dozens of holidaymakers have been involved in 'balconing' incidents resulting in serious injuries and sometimes deaths.
Magaluf on the Balearic island of Majorca has introduced bylaws that will impose fines on anyone caught attempting to climb out of balconies.