Gear Ratio Speed Calculator
Check speed, RPM, and shift drop from tire size, final drive, and gear ratio.
Calculator guide
Understanding speed, RPM, and shift drop
This calculator answers how fast the car will travel at a given engine speed, what RPM a chosen road speed requires, and where engine speed should land after an upshift.
Inputs and calculation
- Engine RPM or vehicle speed, depending on the answer you select.
- Current gear ratio, final-drive ratio, measured tire diameter, and the next gear ratio.
Speed in mph = (RPM × tire diameter in inches) ÷ (gear ratio × final drive × 336). The RPM calculation rearranges that relationship.
RPM after the shift = shift RPM × (next gear ratio ÷ current gear ratio).
How to interpret the result
- Use speed at RPM to check shift points or finish-line gearing. Use the after-shift RPM to see whether the engine returns to the useful part of its power band.
Assumptions and limits
- The result assumes no clutch, converter, or tire slip and a fixed rolling diameter. Tire growth and loaded radius can make real speed differ.
- Use ratios from the actual transmission and differential; advertised tire diameter is only a starting point.
Example: fourth-to-fifth shift
At 7,000 RPM with a 1.35 gear, 4.10 final drive, and 25.6-inch tire, calculated speed is about 96.4 mph. Shifting to a 1.03 gear should land near 5,341 RPM.
Save gearing decisions with the setup
TuneWorks keeps gearing notes with the vehicle, setup, sessions, and logs so each change has context.