Free gear ratio calculator

Gear ratio calculator RPM, speed and re-gearing

Enter the axle ratio, transmission gear and tire size to get engine RPM at a speed or the speed at an RPM, then see what axle ratio puts RPM back where it was after a tire size change.

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What do you want to work out?

Where 336 comes from: a mile is 63,360 inches and an hour is 60 minutes, so tire RPM = mph × 63,360 ÷ 60 ÷ (π × diameter) = mph × 336.13 ÷ diameter. The calculator uses 336.135.

Engine speed-
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Assumes no slip: a manual transmission or a locked torque converter. With the converter unlocked the engine turns faster than this. Tire sizes are nominal; a loaded tire is a little shorter.

In short

Engine RPM = mph × axle ratio × transmission gear ratio × 336 ÷ tire diameter in inches. The 336 is 63,360 inches per mile ÷ 60 minutes ÷ π, or 336.13. At 65 mph with 3.73 gears, a 1.00 top gear and 31-inch tires, the engine turns about 2,629 RPM. To keep that RPM on 35-inch tires, multiply 3.73 by 35 ÷ 31: about 4.21.

How it works

How to use the Gear Ratio Calculator

1

Pick what to work out

Engine RPM at a speed, the speed at an RPM, or the axle ratio for new tires.

2

Enter ratios and tire

Axle ratio, the transmission gear (1.00 for direct drive, below 1 for overdrive) and the tire diameter or size.

3

Read the table

RPM from 30 to 80 mph, speed from 1,500 to 6,000 RPM, or RPM before and after re-gearing.

What it does

What the gear ratio calculator works out

RPM at any speed

Engine and tire RPM at your speed and at six common speeds.

Speed at any RPM

Road speed in mph and km/h for a given engine speed and gear.

Re-gear for bigger tires

The axle ratio that gives the same RPM on the new tire, and the RPM drop if you keep the old gears.

Tire size or diameter

Type 31, 265/70R17 or 33x12.50R15; sizes are converted to a nominal diameter.

The math shown

Every result shows the formula with your numbers, including where 336 comes from.

Engine RPM
mph × axle ratio × gear ratio × 336.13 ÷ tire diameter (in)
Speed
RPM × tire diameter ÷ (axle ratio × gear ratio × 336.13)
The constant
63,360 in per mile ÷ 60 min per hour ÷ π = 336.135
Re-gear
new axle ratio = old ratio × new tire diameter ÷ old tire diameter
Assumes
No slip: a manual or a locked torque converter; nominal tire diameter
FAQ

Questions about the gear ratio calculator

How do you calculate engine RPM from speed and gear ratio?
Multiply the speed in mph by the axle ratio, the transmission gear ratio and 336, then divide by the tire diameter in inches. 65 × 3.73 × 1.00 × 336.13 ÷ 31 = about 2,629 RPM.
Where does the 336 in the gear ratio formula come from?
It converts miles per hour into tire revolutions per minute. A mile is 63,360 inches, so a tire covers mph × 63,360 ÷ 60 inches a minute; dividing by its circumference, π × diameter, gives mph × 336.13 ÷ diameter.
What gear ratio do I need for bigger tires?
Multiply the current axle ratio by the new tire diameter divided by the old one. Going from 31 to 35 inch tires on 3.73 gears needs about 4.21 to keep the same RPM; pick the closest ratio offered for your axle.
Does overdrive change the calculation?
Yes: use the gear's ratio, such as 0.70 for a 30% overdrive. At 70 mph, 4.10 gears, 0.70 overdrive and 33-inch tires give about 2,046 RPM.
Why is my tachometer higher than the result?
With an automatic, the torque converter slips until it locks, so the engine turns faster than the formula says. A loaded tire is also a little shorter than its nominal size.
Keep going

Guides and features for this

Tire size calculatorBayRobinSpeedometer error calculatorBayRobinTransmission shopsBayRobinBrake and tire shopsBayRobinAll guides on running an auto repair shopTopic guides
Sources

Where the formulas and rules come from

Checked October 2026. Estimates from nominal tire diameters, assuming no converter slip. Check against a scan tool or GPS speed.

Link to this tool

Useful for your team or your readers? Paste this into a blog post, forum or resource page.

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