# Calculate Shift RPMs

It is important to shift the gears at the proper time. If the gears are shifted at a higher RPM than necessary, it increases the vehicleâ€™s acceleration speed, which can cause serious damage to the engine. Calculate the optimal RPM for shifting using the gear ratio. Divide the next highest gear ratio by the current gear to determine the proper RPM to shift.

Count the number of teeth on the idler gear, which is the larger gear. In this example, the idler will have 40 teeth.

Determine the number of teeth on the pinion or the smaller gear. In this example, the pinion will have 20 teeth.

Form a fraction with the idler gearâ€™s teeth number on top and the pinionâ€™s teeth number on the bottom. In this example, it is 40/20. Reduce it to the simplest form, which is 2/1 or 2. The gear ratio is calculated by putting this number over 1; it is 2/1, or 2:1.

Divide the current gearâ€™s ratio, 2:1, by the next gearâ€™s ratio. In this example, the next gearâ€™s ratio is 3:1. This will tell you how many RPMs will drop when shifting from first to second gear at 6000 RPM. It will drop by one-third, or to 4000 RPM. In this example, the optimal shift point for shifting from first to second gear is between 6400 and 6800 RPMs. If the RPMs are over 6000, it will cause the engine to accelerate too much and if it is below 4000, it will not be enough.

Watch the tachometer, which is a dial on the carâ€™s dash that measures RPM. A typical tachometer has a range of numbers from 1 to 9 that relate to the RPM. Multiply the number marked on the tachometer by 1000 to calculate the engineâ€™s RPM. Once the tachometer reaches the desired RPMs for shifting gears, you will know when to shift.

Tips

Look for a red warning light or numbers highlighted in red on the tachometer. If this happens, you will need to shift up immediately or the engine will over-rev and could cause an oil leak or a fire.

As you get better at driving a manual transmission vehicle, you can just listen for when the engine is turning faster and you will know when to shift.