What does gear ratio mean?

Answer 1

It's the difference in rotational speed from the input to the output shaft. Most people are talking about rear-end gears of rear-wheel-drive cars when they refer to gears ratios. For instance, a common pickup gear ration is 3.55, which means that the input (drive) shaft turns just over 3-1/2 times for every time the wheels turn once. When you see numbers such as 3.55, it's actually 3.55:1, but the ":1" is dropped, since it's the same reference for all ratios. "Tall" or "fast" gears are better for highway mileage, fuel economy, and creature comfort, and are normally in the 2.75 to 3.23 range. "Deep" or "quick" gears are better for acceleration owing to their increase of rear-wheel torque. These usually run around 3.50 and higher (ratio number). Note that as the ratio number goes up, the car's relative speed goes down, since the engine can only turn so fast. With the later overdrive transmissions, you can have the best of both worlds by running deeper gears for better acceleration, then making up the difference once in overdrive by slowing the engine down for less interior noise and improved fuel mileage.