Practical tools for measured work
Speed to RPM Converter
Convert road speed to engine or driveshaft RPM, or RPM to road speed, from tire diameter and one gear ratio.
Selected gear x final drive, or driveshaft ratio for a 1:1 direct check
RPM = speed (mph) x gear ratio x 336.14 / tire diameter (in). The "336.14" constant is the exact form of the commonly-rounded 336.
How it works
What this converts: enter a tire diameter and an overall gear ratio (the selected gear's ratio multiplied by the final-drive ratio, or 1 for a direct driveshaft-to-wheel check), then either a road speed to find the resulting RPM, or an RPM to find the resulting road speed.
Formula. RPM = MPH × gear ratio × (336.14) ÷ tire diameter (inches). The constant comes from converting miles-per-hour into inches travelled per minute (63,360 ÷ 60) and dividing by π to turn that into wheel revolutions - it's the precise version of the 336 constant used in most driveline reference charts.
Example. A 26-inch tire with a 3.55 overall ratio at 60 mph turns about 2754 RPM.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Solve for
- RPM from speed
- Tire diameter
- 26 in
- Overall gear ratio
- 3.55
- Road speed
- 60 mph
- Engine/driveshaft RPM
- 2500 rpm
Primary result: RPM: 2754 rpm.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This is drivetrain gearing math only, not a tire-fitment check. It assumes no wheelspin, converter or clutch slip, and that the tire diameter entered is the actual rolling diameter rather than the nominal sidewall figure. For a full multi-gear plan across an entire transmission, use the Gear Ratio, Road Speed & RPM Planner instead.
Frequently asked questions
What gear ratio should I enter for a driveshaft RPM check?
Enter 1. That treats the ratio input as a pass-through, so the result is wheel/driveshaft RPM directly rather than engine RPM through a transmission gear.
Why does a bigger tire lower RPM at the same speed?
A larger tire covers more ground per revolution, so it needs fewer revolutions - and therefore lower RPM through a fixed gear ratio - to cover the same distance in the same time.
Should I use nominal or measured tire diameter?
Measured (loaded/rolling) diameter gives a more accurate result. Nominal sidewall diameter is close but real tires rarely match it exactly.
Related tools
Gear & RPM Planner
Compare road speed and engine RPM across six transmission gears using tire diameter and axle (final-drive) ratio.
Tire Size Calculator
Compare two tyre sizes side by side - diameter, sidewall, circumference and speedometer effect.
Wheel Offset Calculator
Compare a current and proposed wheel/tyre setup and see exactly how the geometry changes.
Alternative Tire Sizes
Find standard tire sizes, including different wheel diameters, that keep overall diameter within a chosen tolerance of a starting size.