How RPM, Speed, and Gearing Work Together
Engine RPM, vehicle speed, transmission gearing, axle ratio, and tire diameter are all connected. Changing any one of these can change the relationship between engine speed and road speed.
A numerically higher transmission or final-drive ratio generally causes the engine to turn faster at a given road speed. A numerically lower ratio generally reduces engine RPM at the same speed. Tire diameter also matters because a larger-diameter tire travels farther with each revolution than a smaller tire.
Understanding Transmission and Final-Drive Ratios
A transmission ratio describes the relationship between the transmission's input and output speeds. A 1.00:1 gear is commonly referred to as direct drive. A ratio below 1.00:1, such as 0.75:1, is an overdrive ratio.
The final-drive ratio is the axle or differential ratio. For example, a 3.73:1 final-drive ratio means the driveshaft turns approximately 3.73 times for one rotation of the driven wheels, assuming the transmission output is directly connected to the driveshaft.
The transmission ratio and final-drive ratio work together to create the overall drive ratio.
Why Tire Diameter Matters
Tire diameter directly affects the distance the vehicle travels for each wheel revolution. Installing taller tires can reduce engine RPM at a given road speed, while shorter tires can increase RPM.
The size printed on the sidewall of a tire can be used to estimate its diameter, but the calculated diameter is nominal. Actual rolling diameter can vary because of tire design, tread wear, inflation pressure, vehicle load, and other factors.
Using Gear Comparison
Comparing several transmission gears at the same road speed can show how much engine RPM changes from one gear to another. This can be useful when comparing transmissions, axle ratios, tire sizes, or possible drivetrain combinations for a project vehicle.
Remember that the calculated RPM and speed values are theoretical. Real-world results can differ because of factors such as torque-converter slip, clutch slip, tire dimensions, and drivetrain behavior.