One gear's ratio, from shaft speeds
With the vehicle safely on a lift, wheels off the ground, run the engine in the gear you want to measure. Compare input shaft speed (engine or torque converter) against output shaft speed (the driveshaft) and the ratio follows directly — no disassembly needed.
The output shaft is turning slower than the input, meaning this gear is multiplying torque at the cost of speed — typical of the lower, numerically higher gears.
Got the final drive ratio too? Multiply them into the overall ratio that actually relates engine speed to road speed.
Getting a number you can act on
- 01Get the vehicle safely on a lift
Wheels need to be free to rotate for the output shaft to turn at all. Never attempt this measurement with the vehicle on the ground.
- 02Select the gear you want to measure
Manual transmissions: hold the clutch, select the gear, ease it out slowly with the engine idling. Automatics: the same test uses the torque converter's input speed, which is closer to but not identical to engine speed.
- 03Read input and output shaft speed
Input is engine speed on a manual, or converter input on an automatic. Output is driveshaft speed — measurable with a tachometer probe or an OBD-II data logger on many vehicles.
- 04Divide input by output
That gives the gear ratio. A result above 1.0 is an underdrive gear (typical of the lower gears); a result below 1.0 is an overdrive.
What the calculator is actually doing
Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.
ratio = input shaft speed ÷ output shaft speedNumerically identical to counting the teeth on the gear pair actually engaged, but measurable without disassembly.
Why gear ratio and axle ratio get confused
Both are expressed the same way — a number to one — and both multiply torque while dividing speed, which makes them easy to conflate. The difference is where each one lives and how many of them a vehicle has.
A transmission has one gear ratio per gear — five, six, eight or ten of them on a modern gearbox, one active at a time depending on which gear is selected. The final drive (axle) ratio is fixed: one number, set by the ring and pinion in the differential, that applies in every gear equally.
This tool measures the gear ratio specifically — whichever single gear is engaged during the test. Combine it with the axle ratio, using the overall gear ratio calculator, to get the number that actually relates engine speed to road speed.
Reading what the result tells you
- A ratio above about 2.5 is almost certainly first gear — high torque multiplication, low top speed in that gear
- A ratio between roughly 1.2 and 1.8 typically indicates a middle gear
- A ratio at or near 1.00 is a direct-drive gear — nothing multiplied or divided, common as fourth on a five-speed or a mid-range gear on a wider-ratio box
- A ratio below about 0.95 is an overdrive — the output shaft outruns the input, lowering engine speed for a given road speed
- Modern 8 and 10-speed automatics often have two or more overdrive gears, sometimes down near 0.6:1 in top gear
- None of these ranges are fixed rules — they describe typical practice rather than a specification
Transmission Gear Ratio Calculator FAQ
How do I calculate transmission gear ratio?+
Divide input shaft speed by output shaft speed for the gear engaged. A 3,500 rpm input against a 1,000 rpm output is a 3.5:1 ratio.
What is the difference between gear ratio and axle ratio?+
Gear ratio is one of several values inside the transmission, one active per gear selected. Axle ratio is a single fixed value in the differential that applies in every gear.
How do I measure gear ratio without removing the transmission?+
On a lift with wheels free to turn, compare input shaft speed (engine, or converter input on an automatic) against output shaft (driveshaft) speed while the gear is engaged.
What does a 1:1 gear ratio mean?+
Direct drive — the output shaft turns at exactly the same speed as the input, so that gear itself neither multiplies torque nor increases speed.
Is overdrive always the highest gear?+
Usually, but not always — some transmissions have more than one overdrive ratio, particularly modern 8 and 10-speed automatics with several gears below 1:1.
Standards and references behind these figures
The arithmetic on this page is fixed, but the boundaries and conventions around it come from published standards and manufacturer guidance. These are the documents they come from, so you can check them rather than take them on trust.
01Ford Performance — dynamometer testing and engine performance tech tipsManufacturer guidance on dyno correction and how quoted power figures are arrived at.↗Calculated, not measured. These figures assume nominal tire sizes and the ratios you entered. Real rolling radius changes with load, pressure and wear, and a vehicle's actual axle ratio is not always the one on the door sticker. Verify the ratio on the differential tag before spending anything on gearing.