Gear Ratio Calculator
Turn ring and pinion teeth counts into a gear ratio, find engine RPM at a given speed (or the speed at a given RPM), and see how much a tire-size change effectively re-gears your axle.
Ring & pinion teeth → gear ratio
Gear ratio
3.73:1
Speed ↔ engine RPM
Engine RPM at 65 mph
2858
Effective ratio after a tire-size change
A taller tire spins fewer times per mile, which acts like a numerically lower axle ratio even though the differential never changed.
Old tire
New tire
Old diameter
30.47"
New diameter
32.71"
Effective ratio
3.47:1
How it's calculated
Gear ratio: ring gear teeth ÷ pinion gear teeth. A 41-tooth ring gear with an 11-tooth pinion is 41 ÷ 11 = 3.73:1 -- a very common axle ratio.
Engine RPM at speed: RPM = (MPH × 336 × axle ratio × transmission ratio) ÷ tire diameter (inches). At 65 mph, a 3.73 axle ratio, a 1:1 transmission ratio (direct drive, like a typical highway top gear), and a 28.5" tire: (65 × 336 × 3.73 × 1) ÷ 28.5 = 2,858 RPM. Solving the same equation for MPH instead runs it in reverse.
Effective ratio after a tire change: actual ratio × (old tire diameter ÷ new tire diameter). Going from a 245/75R16 (30.47" diameter) to a 285/70R17 (32.71") on a 3.73 axle: 3.73 × (30.47 ÷ 32.71) = 3.47:1 effective -- the axle itself is still 3.73, but the engine now has to work like it's geared to 3.47 to hold the same speed.
These are the standard drivetrain formulas published by gear manufacturers and used in every widely available gear ratio calculator -- not a substitute for a real fitment check before you order parts.
Frequently Asked Questions
How do I find my ring and pinion teeth counts without pulling the cover?
Most axle tags (a small metal tag under a cover bolt or stamped on the housing) list the ratio directly, like "3.73" or "41-11" (41 ring teeth, 11 pinion teeth). If the tag is missing or unreadable, counting teeth with the cover off is the only fully reliable method -- ratio stickers and build sheets can be wrong after an axle swap.
Where does the 336 constant in the RPM formula come from?
It converts miles-per-hour into tire revolutions per minute: 1 mph equals 1,056 inches per minute (5,280 ft × 12 in ÷ 60 min), and dividing that by a tire's circumference (π × diameter) gives 336.13 ÷ diameter, rounded to 336 by every published version of this formula. It's exact for the unit conversion; the diameter you enter is what introduces any real-world rounding.
Why would I want to know my "effective" gear ratio instead of my real one?
Your differential's actual ratio never changes when you put on bigger tires, but the engine has to turn faster to move the same distance -- functionally identical to running a numerically lower (taller) gear. If your bigger tires made throttle response feel lazier, the effective ratio number shows how much taller you effectively went, which is what a re-gear would need to correct.
What tire diameter should I enter for the RPM calculator?
Use the tire's actual overall diameter, not the wheel diameter. If you only have the sidewall size (like 285/70R17), run it through the effective-ratio section below first, or the tire size calculator, to get the real diameter in inches.