Transmission Gear Calculator (speed in every gear, rpm drop on upshift)

Understanding the transmission gear calculator — concept and principle

This calculator turns a gearbox's ratios, the final drive ratio and the tyre size into road speed in every gear at a chosen engine speed. It also shows the engine speed at 100 km/h in top gear, the rpm drop after shifting from first to second, and the ratio spread.

Each wheel turn moves the car one tyre circumference.

How it is calculated

Overall ratio = gear ratio × final drive ratio

Wheel rpm = engine rpm ÷ overall ratio

Speed (km/h) = wheel rpm × tyre circumference (m) × 60 ÷ 1,000

Tyre diameter (mm) = rim × 25.4 + 2 × width × aspect ÷ 100; circumference = π × diameter

Rpm after the 1→2 upshift = rpm × second ratio ÷ first ratio.
Ratio spread = first-gear ratio ÷ top-gear ratio. Speeds are also shown in mph (1 mile = 1.609344 km).

The principle

Gears trade turning speed for force. A ratio of 3.54 means the engine turns 3.54 times for each turn of the gearbox output, and the final drive divides it again before the wheels. Each wheel turn moves the car one tyre circumference. In a given gear, speed and engine rpm are therefore locked in proportion, so the speed at the redline is the most that gear can reach. When you upshift at the same road speed, the engine speed falls in the ratio of the two gears. A wide spread gives strong starts and quiet cruising; close ratios keep the engine near its best rpm.

Worked example

Default inputs: ratios 3.54, 2.05, 1.37, 1.03, 0.85, 0.72; final drive 3.94; tyre 225/45R17; 6,500 rpm.

Tyre diameter = 431.8 + 202.5 = 634.3 mm; circumference = 1.9927 m.
First gear: 6,500 ÷ (3.54 × 3.94) = 466.0 wheel rpm; × 1.9927 × 60 ÷ 1,000 = 55.7, shown as 56 km/h · 35 mph.
The other gears give 96, 144, 192, 232 and 274 km/h (60, 89, 119, 144 and 170 mph).
Top gear at 100 km/h: 100,000 ÷ 60 ÷ 1.9927 × 0.72 × 3.94 = 2,373 rpm.
After the 1→2 upshift at 6,500 rpm: 6,500 × 2.05 ÷ 3.54 = 3,764 rpm; spread = 3.54 ÷ 0.72 = 4.92.

How to read the result

The per-gear figures are what the gearing allows, not what the car can reach. In top gear the speed at the redline is usually far beyond what engine power and air drag permit. The cruise rpm at 100 km/h shows how relaxed the car is on the motorway. If the rpm after an upshift falls below the engine's useful band, the gears are too far apart for hard acceleration. A shorter final drive (larger number) raises every rpm figure and lowers every speed.

Common mistakes

Leaving out the final drive ratio.
Using the rim size as the tyre diameter.
Treating the redline speed in top gear as the car's top speed.
Forgetting that a tyre change alters every gear speed.

Keep in mind: The figures ignore tyre slip, tyre growth at speed and torque-converter slip in automatics. Use them for comparison, not as legal top speeds or for tuning without professional advice.

Frequently asked questions

What does a 4.10 final drive do compared with 3.73?

The higher number gives quicker acceleration and higher rpm at any speed, with less top-gear speed per rpm.

Why is the overall ratio in top gear below 1 × final drive?

Top gears of 0.7–0.9 are overdrive: the gearbox output turns faster than the engine to cut cruising rpm.

Sources: ISO 4000-1, Passenger car tyres and rims (tyre size designation) · Gear-train kinematics: wheel speed = engine speed ÷ overall ratio

Checked 9 Nov 2026. Gear-train kinematics (wheel speed = engine speed ÷ overall ratio; distance per revolution = π × tyre diameter) and the metric tyre size designation (ISO 4000-1 / ETRTO: width mm / aspect % / rim inches). The gear set shown is an example. 1 mile = 1.609344 km.

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