Hey Guys:
While the 85% rule may apply pretty much with all pickup trucks, I think we are playing guessing games, which can be potentially very dangerous. There are a couple of new small cars on the market (the new Hybrids, even Honda's) that have "0" listed as their towing capacity.
Find the car's owner's manual. It is a little booklet that usually comes in the glove compartment of a car, and is rarely read by the owner. It will always tell you specifically what the car can tow. On an older car that has lost the manual, check a junkyard for the manual, or the manufacturer's website. Or just get on the phone and call the local Honda (or other brand) dealer.
My 1994 VW Bug (air-cooled, 1600 cc. fuel-injected) can tow 885 pounds, with 55-82 pounds tongue weight. For this weight I have to get a hitch that attatches to the bumper and the transaxle of the Bug!
My wife's 2005 Chevrolet Corsa (1800 cc, 5-speed) can tow 1980 pounds if the trailer has no brakes, and a whopping 900 pounds more if the trailer has brakes! I was amazed! (Most Americans have never seen a Corsa, but it is a clone of the VW Jetta, made in Argentina.)
The real authorised manufacturer-recomended towing capacity also depends on manual or automatic transmision, suspension, body style (hatchback, coupe, sedan, station wagon), engine size, tires, etc.
Most insurance companies will NOT cover you in an accident, if you have exceeded the manufacturer's weight limitations. So the proper, safe thing to do is to play safe, and check the manual for your specific vehicle.
Dr. Steve
Central old Mexico