About this tool
The Engine RPM & Vehicle Speed Calculator links engine speed and road speed through your drivetrain. Given a transmission gear ratio, the final-drive (axle) ratio and the tire diameter, road speed follows MPH = (RPM × tire diameter in inches) / (gear × final drive × 336), where 336 ≈ 63360 / (π × 60) converts inches-per-minute at the wheel into miles per hour. Rearranged, RPM = MPH × gear × final drive × 336 / tire diameter, so you can solve in either direction.
Pick what to solve for — speed from a given RPM, or the cruising RPM at a given road speed — and enter your ratios and tire size. Tire diameter can be entered directly in inches, or typed as a P-metric size like 225/45R17, from which the overall diameter is computed as (section width × aspect / 100 × 2 / 25.4) + rim diameter. This is handy for comparing tall versus short tires or checking highway RPM after a gear or diff change.
Speed can be shown in mph or km/h; the diameter always stays in inches for the tire math and the km/h result is converted from mph. The calculation assumes direct-drive with no torque-converter slip or tire growth, so real-world RPM is usually a touch higher. Everything runs in your browser with no uploads — a quick way to answer 'what's my RPM at 70 in 6th?' before a long drive or a gearing swap.