How do undersized pulleys make a car faster?

Answer 1

Undersized pulleys will turn the Power Steering, alternator, A/C compressor slower that standard sized pulleys therefore using less power from the engine. The freed up power may allow you to drive slightly faster than before. The true benefit for most street cars is better fuel economy. In a high powered race car the benefit can be measured in hundredths or thousandths of a second. The horse power gain has been claimed to be 5 - 15 extra horsepower.