Gear Ratio Calculator: RPM per Gear, Speed at Redline and Shift Drops

Gear ratio calculator: pick a transmission or enter your own ratios, add final drive and tire size, and see engine RPM in every gear, speed at redline and the RPM drop on each shift.

Gear ratio calculator

How the numbers are calculated

Engine speed is road speed multiplied through every reduction between the wheels and the crank - the transmission gear and the final drive - and divided by how far the tire travels per turn:

This is the formula Tremec publishes for its transmissions, where the constant is rounded to 336. Exactly, it is 63,360 inches in a mile divided by 60 minutes and by π (336.135), which is what the calculator uses. Metric tire sizes are converted to a theoretical diameter (wheel + 2 × width × aspect ratio ÷ 100 ÷ 25.4); if your tire maker lists revolutions per mile, use that mode for the closest number.

Where the presets come from

Every preset ratio comes from a manufacturer document: BMW press technical data for the Getrag DCT and the ZF 8HP, BMW training manuals for the DCT, ZF's transmission brochure plus BMW data for the second-generation 8HP, BMW sales brochures and BMW parts catalog data (bmwfans.info mirror) for the E32, E34 and E31 gearboxes, Wikipedia for the ZF 5HP and GM 4L30-E gearbox names, and Tremec's Magnum service manual and TR-6060 product sheet. A few things worth knowing:

There are two DCT gear sets. The E9x M3, 135i (N55), 335is and Z4 sDrive35i/35is use a set with a direct (1.000) 7th gear. The F10 M5, F06/F12/F13 M6, F80 M3, F82 M4 and F87 M2 use a set with a direct 5th and overdrive 6th and 7th. Same family, very different cruise RPM, so check which one your donor came from.

8HP ratios change by generation, not just by torque rating. BMW's first-generation 8HP45, 8HP70 and 8HP90 share one set of forward ratios; the second-generation 8HP50 and 8HP75 start from a 5.000 first gear. Chrysler 300 and Ram 1500 spec sheets list the first-generation ratios. BMW lists third-generation ratios by car rather than by variant name, so those presets are labeled by the car they came from.

What this doesn't account for: real rolling diameter under load, and slip in a torque converter that isn't locked (automatic results assume a locked converter). Speed at redline in top gear is the gearing limit - actual top speed is set by power and drag, and is usually lower.

Driveshaft speed doesn't depend on which gear you're in - see our driveshaft speed calculator. Weighing a DCT against an 8HP? Our DCT and ZF 8HP swap basics guide covers parts, controllers, sourcing and costs.