Work out a gear ratio from tooth counts, the final drive or axle ratio from a ring and pinion, the overall ratio (transmission gear × axle) and the off-road crawl ratio — then solve speed, RPM, ratio or tire size from one another, and see how taller tires throw off your speedometer and what regear restores stock gearing. Enter your own numbers; nothing is looked up and nothing is sent anywhere.
Gear ratio from teeth (driven ÷ drive)
For a ring-and-pinion axle this is the same thing — put the ring on the driven side and the pinion on the drive side. If you already know the ratio you can skip these and just type it into the overall/speed modes.
Optional: transmission gear set
Enter each forward gear's ratio (leave blank for gears you don't have). The table below shows each one combined with the axle ratio.
Overall ratio (transmission gear × axle)
Top gear is 1.00 if direct, below 1.00 if overdrive.
Crawl ratio (low-range total reduction)
Two-wheel-drive? Leave the transfer-case box at 1 and it's just trans low × axle.
Solve for
In a chosen gear — leave the one you're solving for blank
Don't know the overall ratio? Multiply the gear you're in by the axle ratio in the Overall mode first. Don't know the tire diameter? Compute it from a tire size in the Tire-change mode.
Tire diameter — enter directly, or compute from a size
Type a size like 285/75R16 and the diameter fills in automatically. Measured beats nominal — sizes are theoretical, so confirm with a tape if you can.
A gear ratio is just one count divided by another: the driven (output) gear's teeth over the drive (input) gear's teeth. Written ratio = driven ÷ drive, a 41-tooth wheel turned by a 13-tooth wheel is 41 ÷ 13 = 3.15, said “3.15 to 1.” The input turns 3.15 times for every single turn of the output, trading speed for torque. That is the whole formula for gear ratio, and it is the same whether you are looking at a gearbox pair, a chain sprocket or an axle. The one thing to get right is which gear is which — flip them and you get the reciprocal — so this drive ratio calculator keeps the boxes clearly labelled driven and drive.
Final drive, axle, differential and ring-and-pinion — one ratio, many names
The final drive ratio is the reduction in the axle, and it goes by a pile of names: axle ratio, differential gear ratio, rear-end gear ratio, or ring-and-pinion ratio. They are all the same number, ring teeth ÷ pinion teeth. A 41-tooth ring on a 13-tooth pinion is 3.15:1; a 37/9 set is about 4.11:1. If the differential tag or your records already give the ratio, type it straight in — you don't have to count teeth. This is the axle ratio calculator and ring-and-pinion gear ratio calculator side of the tool, and the figure it gives is one of the two numbers you need for the overall ratio.
Overall (total) ratio: the number that actually sets your RPM
This is where most people go wrong. The overall ratio is the transmission gear ratio multiplied by the axle ratio: overall = transmission gear × axle. It is the figure that decides engine RPM at a given speed — not the axle on its own. A top gear that is direct is 1.00:1; an overdrive top gear is below 1.00, often 0.70 to 0.85. So a 3.73 axle with a 0.73 overdrive gives an overall of only 2.72, and if you drop the axle alone into the speed formula your RPM comes out wildly high. Always combine the gear you are actually in with the axle. The transmission-set table in the first mode does this for every gear at once.
Crawl ratio for low-range four-wheel drive
Off-road, the number that matters at walking pace is the crawl ratio — the total reduction with everything stacked in low range: crawl = transmission low × transfer-case low × axle. A 4.71 first gear, a 4.0:1 low-range case and a 4.10 axle multiply to about 77:1. The higher it is, the slower and more controllable the vehicle creeps over obstacles at idle, with less brake and clutch work. Builds commonly aim somewhere between 50:1 and 90:1 depending on tire size and use. Enter your own three figures and the tool multiplies them.
Speed ↔ RPM in any gear
The standard relationship between road speed, engine RPM, gearing and tire size is:
The 336 is a unit constant: 63,360 inches in a mile, divided by 60 minutes, divided by π. The ratio in these formulas is the overall ratio (transmission gear × axle), which is exactly the trap that produces wrong answers when only the axle is used. The Speed ↔ RPM mode rearranges the formula for whichever box you leave blank, so it works as a gear ratio speed calculator, a gear ratio RPM calculator, or even a top-speed gear ratio calculator if you put in your redline. Typical highway cruise sits around 1,800–2,500 RPM in top gear.
What bigger tires do to gearing and your speedometer
A taller tire rolls further per revolution, so it behaves like a numerically lower axle — it lowers your effective gearing, which is why a truck feels slower and turns fewer RPM after a tire upsize. The effective ratio is:
effective ratio = (old diameter ÷ new diameter) × original ratio actual speed = indicated speed × (new diameter ÷ old diameter) % speedometer error = (new − old) ÷ old × 100
Because the wheel turns slower for the same road speed, the speedometer reads low after fitting bigger tires (and reads high after fitting smaller ones), and the odometer, cruise control and automatic shift points drift with it. This is the tire size gear ratio calculator and speedometer error side of the tool: enter old and new diameters — directly or computed from a tire size — and it gives the effective gearing and the true speed at any indicated reading.
Regearing to restore stock gearing
To put the engine back where it was for a given speed, you regear by the same proportion the tire grew:
new ratio = stock ratio × (new diameter ÷ old diameter)
With a 3.73 axle and a jump from a 28-inch to a 33-inch tire, 3.73 × 33 ÷ 28 ≈ 4.40, so a 4.40 — or the nearest you can buy, often 4.56 — restores roughly stock performance and cruise RPM. The tool gives the exact figure and you round to an available ratio.
Tire diameter from a tire size
If you only know the sidewall size rather than a measured diameter, the calculator computes it:
For a 285/75R16, that's 16 + 2 × (285 × 0.75) ÷ 25.4 ≈ 32.8 inches. This is a theoretical, unloaded figure — real mounted height varies with wheel width, pressure, load and brand, so a measured diameter is always better when you have one.
A worked example
You're cruising at 70 MPH in a 0.73 overdrive top gear behind a 3.73 axle, on 28-inch tires.
Overall ratio = 0.73 × 3.73 = 2.72 (using the axle alone, 3.73, would overstate RPM by a third).
A note on spelling and units for readers outside the US: “tire” and “tyre” are the same thing, “gearing” and “gear ratio” are used interchangeably, and the tool will work in inches and MPH or in millimetres and km h⁻¹. The arithmetic is identical; only the labels differ.
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Frequently asked questions
What is the formula for gear ratio?
Gear ratio is the driven gear's tooth count divided by the drive gear's tooth count: ratio = driven ÷ drive. A pinion of 13 teeth turning a ring of 41 gives 41 ÷ 13 = 3.15, written 3.15:1, meaning the input turns 3.15 times for one turn of the output. The same rule works for any meshing pair; if you only have rotational speeds, dividing input RPM by output RPM gives the same number. The single most common error is inverting it — dividing drive by driven — so this calculator labels the boxes driven and drive so the larger, slower wheel goes on top.
How do I calculate the final drive or axle ratio from ring and pinion teeth?
The final drive ratio (also called the axle, differential, rear-end or ring-and-pinion ratio) is the ring gear's tooth count divided by the pinion's: ratio = ring ÷ pinion. A 41-tooth ring on a 13-tooth pinion is 3.15:1; a 37/9 set is about 4.11:1. If you already know the printed axle ratio you can type it straight in instead of counting teeth. This is the differential gear ratio calculator side of the tool, and the number it gives is one of the two factors in the overall ratio.
What is the overall (total) gear ratio and why does forgetting the transmission gear give the wrong answer?
The overall ratio is the transmission gear ratio multiplied by the axle ratio: overall = transmission gear × axle. It is the figure that actually sets engine RPM for a given road speed, and the mistake people make is using only one of the two. A direct-drive top gear is 1.00:1, but an overdrive top gear is below 1.00 — commonly around 0.70 to 0.85 — so a 3.73 axle in a 0.73 overdrive gives an overall of just 2.72, not 3.73. Plug only the axle into the 336 speed formula and the RPM comes out far too high. Always combine the gear you are actually in with the axle before working out cruising RPM.
How do I calculate crawl ratio?
Crawl ratio is the total reduction in low range with everything stacked: crawl = transmission low gear × transfer-case low × axle ratio. A 4.71 first gear, a 4.0:1 low-range transfer case and a 4.10 axle multiply to about 77:1. The bigger that number, the slower and more controllable the vehicle is at idle over obstacles. Enter your own three figures and the tool multiplies them; many off-road builds aim somewhere in the 50:1 to 90:1 range depending on tire size and use.
How do I find RPM at a given speed, or speed from RPM?
Use MPH = (RPM × tire diameter in inches) ÷ (336 × overall ratio), and rearrange for any unknown: RPM = (MPH × overall ratio × 336) ÷ tire diameter. The 336 is a unit constant (63,360 inches per mile ÷ 60 minutes ÷ π). The overall ratio must be the transmission gear times the axle, not the axle alone. For example, at 70 MPH with an overall ratio of 2.72 (a 0.73 overdrive on a 3.73 axle) and a 28-inch tire, RPM = 70 × 2.72 × 336 ÷ 28 ≈ 2,286 — a typical highway cruise sits around 1,800 to 2,500 RPM in top gear. The tool can solve for MPH, RPM, the ratio or the tire diameter, whichever you leave blank.
How do bigger tires affect gearing and the speedometer?
A taller tire travels further per revolution, so it lowers your effective gearing the same way a numerically smaller axle would, which is why a vehicle feels slower and cruises at lower RPM after a tire upsize. The effective ratio is the old diameter divided by the new diameter, times the original ratio: G_effective = (d_old ÷ d_new) × G. It also makes the speedometer read slow: actual speed = indicated speed × (d_new ÷ d_old), and the percent error is (d_new − d_old) ÷ d_old × 100. Going from a 28-inch to a 33-inch tire is about an 18 percent error, so an indicated 60 is really about 71.
What axle ratio restores stock gearing after fitting bigger tires?
To put the engine back at its original RPM for a given speed you regear by the same proportion the tire grew: new ratio = stock ratio × (new diameter ÷ old diameter). With a 3.73 axle and a jump from 28-inch to 33-inch tires, 3.73 × 33 ÷ 28 ≈ 4.40, so a 4.40 (or the nearest available, often 4.56) restores roughly stock performance and cruise RPM. This is the regear-for-bigger-tires calculation; the tool gives the exact figure and you round to a ratio you can actually buy.
Why don't you list ratios for specific transmissions, axles or vehicle models?
Because those tables are vendor- and model-specific, they go out of date, and copying them adds nothing you can trust for your own vehicle — the same model line ships with different axles, and a swapped transmission or differential changes everything. So this tool asks you to enter your own ratios or tooth counts instead, and computes every result from the public gearing formulas. Read the printed axle ratio off the differential tag or the door-jamb option codes, count the ring and pinion teeth, or confirm the figure against your tachometer at a known speed, and the answer reflects your actual drivetrain rather than a generic chart.