RPS to RPM Converter for Engine and Wheel Speeds

Reading Crank and Wheel Sensors in Tachometer Units

A crank position sensor, a driveshaft pickup or a wheel-speed sensor reports one thing: how many complete turns the shaft made in one second. Almost nothing else around a vehicle speaks that language. Tachometers are marked in RPM, rev limits are quoted in RPM, and gearing charts and dyno sheets are built the same way. Multiplying the logged revolutions per second by sixty puts a sensor number back into the units everyone else is already using.

Conversion factor: RPM = rev/s × 60. A warm four-cylinder idling at 12.5 rev/s reads 750 RPM on the tachometer, and a 6 500 RPM rev limit is 108.33 rev/s at the crank sensor.

Three Shafts, Three Different Numbers

What the Crank Pickup Reports

The crankshaft turns twice per power stroke on a four-stroke engine, and the crank rate is what the tachometer scales from. Camshaft and distributor pickups spin at half that rate, so double their reading before comparing it with a dash figure.

Wheel Speed Is Its Own Shaft

A road wheel with a 2.00 m rolling circumference turns 13.889 times per second at 100 km/h, or 833 RPM. That figure does not care which gear is selected, which is why stability and anti-lock systems watch the wheel rather than the crank.

Ratios Live in Between

Gearbox ratio multiplied by final-drive ratio is the whole distance between wheel speed and crank speed. Put both ends into the same unit before you divide — mixing rev/s at one end with RPM at the other is where most gearing sums quietly go wrong.

Working Through a Driveline Speed Check

Enter the raw sensor figure

Type the revolutions per second straight from the datalogger or scan tool into the left field. A comma or a dot both work as the decimal mark, so a value pasted out of a log sheet lands cleanly.

Compare it against the dash reading

The RPM figure follows each keystroke, so you can hold a logged trace next to the tachometer and see whether the gauge is telling the truth. Thousands are spaced apart, which starts to matter once you are past four figures.

Carry the number through the ratios

With both ends now in RPM, multiply wheel speed by the gearbox and final-drive ratios to predict crank speed, or divide the other way to see what road speed a given rev limit allows in each gear.

Move it into your gearing sheet

The copy control on either field puts the bare figure on the clipboard with no unit text attached, so it drops into a spreadsheet cell without needing to be tidied up first.

Going the other way: starting from a tachometer reading and need rev/s for a sensor calculation? Press the swap control, or simply type into the RPM field — both boxes accept input and the opposite side follows.

Engine and Wheel Speeds Across the Rev Range

The table runs from idle up into race-engine territory. The last column gives the time one full turn takes — the figure to reach for when choosing a logger sample rate or working out how long a crank sensor has to resolve a missing tooth.

Shaft condition	Revolutions per second	Tachometer reading	Time for one turn
Warm petrol four-cylinder at idle	12.5 rev/s	750 RPM	80 ms
Road wheel at 100 km/h, 2.00 m circumference	13.889 rev/s	833 RPM	72 ms
Motorway cruise in top gear	33.333 rev/s	2 000 RPM	30 ms
Turbodiesel at its rev limit	75 rev/s	4 500 RPM	13.33 ms
Naturally aspirated petrol rev limit	108.333 rev/s	6 500 RPM	9.23 ms
Formula-style race engine	250 rev/s	15 000 RPM	4 ms

Logged Rev/s Becomes a Dash Number

Both boxes stay live, so a whole column of sensor values can be checked against tachometer figures one after another without reloading the page.

Crank Side or Wheel Side on Demand

One press of the swap control reverses the direction, which suits ratio work where you bounce between the wheel end and the engine end of the driveline.

Angular Units for Torque Maths

Both dropdowns are searchable and carry rad/s and deg/s next to the revolution units, which is what a shaft-power sum needs once the speed figure is settled.

Driveline Speed Questions from the Workshop

My crank sensor logs 108.3 rev/s — what should the tachometer show?

Multiplying by sixty gives 6 500 RPM. If the dash reads noticeably higher or lower, suspect the tachometer's pulses-per-revolution setting rather than the sensor. Aftermarket gauges carry a cylinder-count or pulse-divider selector, and getting that wrong scales the entire display by a fixed factor rather than adding a small error.

What is a redline actually protecting?

Mostly the valvetrain and the reciprocating assembly. Valve springs have less time to close a valve as speed climbs, and past a point the valve stops following the cam profile — that is valve float. Piston acceleration and the loads on rod bolts rise with the square of speed, so a 15 000 RPM race engine spinning at 250 rev/s only survives because its stroke is short and its internals are far lighter than a road engine's.

How do I get wheel revolutions per second from road speed?

Convert road speed into metres per second, then divide by the rolling circumference. At 100 km/h a car covers 27.778 m/s; against a 2.00 m circumference that is 13.889 rev/s, or 833 RPM at the hub. Use the loaded rolling circumference rather than the unloaded tyre size — the gap is a few per cent and it lands straight in any speedometer correction.

How do gear and final-drive ratios link engine speed to wheel speed?

Crank RPM equals wheel RPM multiplied by the selected gear ratio and then by the final-drive ratio. That 833 RPM wheel behind a 1.00 direct gear and a 3.70 differential puts the crank near 3 083 RPM. Swapping the differential or fitting a taller tyre shifts cruising revs by exactly the same proportion, which is why both sit in every gearing spreadsheet.

Why is my reluctor wheel pulse rate far higher than the shaft speed?

Because a tone ring emits one pulse per tooth, not one per turn. A common 60−2 crank trigger has 58 teeth present, so an engine idling at 12.5 rev/s generates 725 pulses every second. Divide the raw pulse rate by the tooth count before treating it as a shaft rate, and check that count per sensor — anti-lock rings on different axles often differ.