What does the constant 336 in the gear ratio formula represent?

It converts revolutions and inches into miles per hour. It is (5280 feet × 12 inches) divided by (60 minutes × π), which folds the tyre circumference and the unit conversions into a single number so the formula stays short.

How do bigger tyres change my gear ratio?

They do not change the mechanical ratio, but they raise the effective gearing: a taller tyre travels further per revolution, so engine RPM drops at a given speed and acceleration suffers. A 28 to 33 inch swap is about 18% taller, which is why a numerically higher axle ratio is usually fitted alongside.

What axle ratio do I need after fitting larger tyres?

Multiply the original axle ratio by the new tyre diameter divided by the old one. Going from 28 to 33 inch tyres on a 3.73 axle suggests about 4.40, so 4.56 would be the nearest commonly available gear set.

Is a higher gear ratio number faster or slower?

A numerically higher ratio means more torque multiplication and lower top speed in that gear — quicker acceleration, higher cruising RPM. A numerically lower ratio is an overdrive, giving relaxed highway revs and better economy.

Does this calculator account for drivetrain slip?

No. It assumes no tyre slip and no torque-converter slip, so a manual transmission or a locked converter matches closely while an unlocked automatic will read a little optimistic.

Gear Ratio Calculator

Work out road speed at a given RPM, the RPM needed to hold a speed, and gear ratios from tooth counts — with tyre size accounted for.

The Gear Ratio Calculator runs entirely in your browser. Ratios, tyre sizes and speeds you enter are used only for the calculation on your device.

Open the Tire Size Calculator

About Gear Ratio Calculator

Gear Ratio Calculator ties together the four numbers that decide how fast a car travels at a given engine speed: the transmission gear ratio, the final drive ratio, the tyre diameter and the RPM. The relationship is MPH = RPM × tyre diameter ÷ (gear × axle × 336), where the constant folds the tyre circumference and the unit conversions into one figure. Enter a P-metric tyre size such as 265/70R17 and the overall diameter is worked out for you. A tyre-swap section shows the percentage change in effective gearing when you fit taller rubber and suggests the axle ratio that would restore the original numbers — the calculation behind most re-gear decisions after a lift kit.

Features

Road speed at a given RPM in both mph and km/h
Engine RPM required to hold a chosen road speed
Gear ratio from drive and driven tooth counts
Overall drive ratio from transmission gear × final drive
Tyre diameter derived from a P-metric size like 265/70R17
Tyre-swap comparison showing the percentage gearing change
Suggested replacement axle ratio to restore original gearing
All arithmetic runs on your device

How to use the Gear Ratio Calculator

Pick whether you are solving for speed, for RPM, or for a ratio from tooth counts
Enter the transmission gear ratio and the final drive (axle) ratio
Enter the tyre size — section width, aspect ratio and rim diameter
Read the result, then use the tyre-swap box to check a different diameter

Example

Input

2000 rpm, 1.00 gear, 3.73 axle, 28 in tyre

Output

44.7 mph (71.9 km/h), overall ratio 3.73:1

Direct-drive top gear with a 3.73 axle — the classic highway-cruise calculation.

Common errors & troubleshooting

The calculated speed is much higher than the speedometer reads. — Check the tyre diameter. Speedometers are calibrated for the factory tyre size, so fitting taller tyres makes the real speed higher than the indicated one — which is exactly what the tyre-swap section quantifies.
Unsure which number is the gear ratio and which is the final drive. — The gear ratio is the transmission's ratio in the gear you are in (often 1.00 in direct drive and below 1.00 in overdrive). The final drive is the differential ratio, typically between 3 and 4.5.
The result is blank after entering a tyre size. — All three tyre numbers are needed — section width in millimetres, aspect ratio as a percentage, and rim diameter in inches. A size like 265/70R17 supplies them in that order.
The tooth-count ratio looks inverted. — A gear ratio is driven teeth divided by drive teeth. An 11-tooth pinion turning a 41-tooth ring gear is 3.73:1, not 0.27:1.

Frequently asked questions

What does the constant 336 in the gear ratio formula represent?: It converts revolutions and inches into miles per hour. It is (5280 feet × 12 inches) divided by (60 minutes × π), which folds the tyre circumference and the unit conversions into a single number so the formula stays short.
How do bigger tyres change my gear ratio?: They do not change the mechanical ratio, but they raise the effective gearing: a taller tyre travels further per revolution, so engine RPM drops at a given speed and acceleration suffers. A 28 to 33 inch swap is about 18% taller, which is why a numerically higher axle ratio is usually fitted alongside.
What axle ratio do I need after fitting larger tyres?: Multiply the original axle ratio by the new tyre diameter divided by the old one. Going from 28 to 33 inch tyres on a 3.73 axle suggests about 4.40, so 4.56 would be the nearest commonly available gear set.
Is a higher gear ratio number faster or slower?: A numerically higher ratio means more torque multiplication and lower top speed in that gear — quicker acceleration, higher cruising RPM. A numerically lower ratio is an overdrive, giving relaxed highway revs and better economy.
Does this calculator account for drivetrain slip?: No. It assumes no tyre slip and no torque-converter slip, so a manual transmission or a locked converter matches closely while an unlocked automatic will read a little optimistic.

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