The serpentine belt is one of those little innovations that never really gets its due. While many Luddites resistant to change bemoaned the fancy new belts when they were introduced, it didn't take long for them to realize that serpentine belts didn't fly off at high rpm, they were far easier to change, didn't wear out as quickly, and even helped to boost fuel economy and horsepower by requiring less mechanical tension. Unsung it might be, but the serpentine belt really is a small marvel of engineering.
The 1991 F-150 used several different length belts, depending on the engine and accessories specified. All used six ribs, were 0.82 inches wide and 0.17 inches thick. The shortest, 91-inch belt saw use on V-8 models, and the rest were used on six-cylinders. The shortest of these, used on trucks without air conditioning, was 93.5 inches. The longest was 98.03 inches, and in between was a belt measuring 97.05 inches. As of 2014, Dayco makes the belts you'll find at almost every parts store, but Autotrust and Replacement brands are also available. The 91-inch V-8 belt is very common, and will run you about $34 and change. Dayco-brand V-6 belts cost $30 to $34, and others can run as much as $41 to $47 a piece.
