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Walk-through

How to Use This Calculator

3 steps

1

Find the ratio from tooth counts

On the Gear Ratio tab, enter the number of teeth on the drive (input/pinion) gear and the driven (output/ring) gear. The ratio is the driven teeth divided by the drive teeth — a 13-tooth pinion turning a 39-tooth ring gear is 3.00:1. The result updates instantly and tells you whether it is a reduction, a 1:1 direct drive, or an overdrive.

2

Turn the ratio into speed and RPM

Switch to the Speed ↔ RPM tab. Enter your transmission gear ratio, the final drive (axle) ratio, and your tire size, then pick a mode: "Speed at RPM" shows how fast you are going at a chosen engine RPM, while "RPM at speed" shows the engine speed needed to hold a chosen road speed. Tap a vehicle preset to fill realistic numbers in one click, and toggle mph or km/h.

3

Compare every gear at once

Open the Multi-Gear Table tab, set your final drive, tire size, and a reference engine RPM (e.g. redline), then list each transmission gear ratio. The table shows the road speed each gear produces at that RPM, highlighting the fastest. Use the CSV button to download the table, or the Share button to copy a link that restores your inputs.

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Reference

Formula & Methodology

4 formulas

Gear ratio

ratio = driven teeth ÷ drive teeth

The gear ratio is the number of teeth on the output (driven) gear divided by the number on the input (drive) gear. A ratio above 1 multiplies torque and reduces output speed (a reduction); a ratio below 1 increases output speed at the cost of torque (an overdrive).

Vehicle speed at RPM

mph = (RPM × tire circumference in miles × 60) ÷ (gear ratio × final drive)

Engine RPM is divided by the total ratio (gear ratio × final drive) to get wheel RPM, then multiplied by the tire's circumference in miles and by 60 (minutes per hour) to convert to miles per hour. For km/h, multiply mph by 1.609344.

Engine RPM at speed

RPM = (mph × gear ratio × final drive) ÷ (tire circumference in miles × 60)

The inverse of the speed formula: wheel RPM (mph ÷ circumference ÷ 60) is multiplied by the total ratio to get the engine RPM needed to hold that road speed in the chosen gear.

Tire diameter from a P-metric size

diameter (in) = rim (in) + 2 × (width mm × aspect% ÷ 100) ÷ 25.4

For a size like 225/45R17, the sidewall height is 225 × 0.45 = 101.25 mm. The overall diameter is the 17-inch rim plus two sidewalls: 17 + 2 × 101.25/25.4 ≈ 24.97 in. Circumference is π × diameter.

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Glossary

Key Terms Explained

6 terms

Gear ratio The ratio of driven (output) gear teeth to drive (input) gear teeth. Higher numbers mean more torque multiplication and lower output speed; lower numbers mean the opposite. Usually written like 3.55:1.

Final drive ratio The fixed reduction in the differential (or motorcycle final sprocket), applied after the transmission. Total reduction = transmission gear ratio × final drive ratio. Typical car final drives run 2.7–4.3:1.

Overdrive A gear ratio below 1.0, where the output shaft turns faster than the input. Top gears in modern transmissions are overdriven (e.g. 0.7:1) to keep highway RPM and fuel use low.

Tire circumference The distance the tire travels in one revolution, equal to π times its overall diameter. It sets how far the car moves per wheel turn, so it directly scales speed per RPM.

P-metric tire size The standard sidewall format, e.g. 225/45R17: section width in millimetres (225), aspect ratio as a percentage of width (45), and rim diameter in inches (17). Used to derive overall diameter.

Redline The maximum safe engine RPM. In a tall gear the RPM needed for a given speed may exceed redline, meaning that speed is unreachable in that gear — the calculator flags this.

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Scenarios

Real-World Examples

3 worked examples

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Differential swap

Ring-and-pinion ratio

Drive (pinion) teeth 13 Driven (ring) teeth 39

39 ÷ 13 = 3.00:1 — a reduction. The axle turns the wheels one-third as fast as the driveshaft but with about three times the torque. A numerically higher ring-and-pinion (say 41/11 = 3.73:1) would quicken acceleration and towing pull but raise cruising RPM and lower top speed.

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Highway cruiser

RPM at 70 mph in top gear

Gear ratio 0.70 Final drive 3.55 Tire size 225/45R17 Speed 70 mph

With a 24.97 in tire (6.53 ft circumference = 0.001238 miles) and a total ratio of 0.70 × 3.55 = 2.485, holding 70 mph needs about 2,340 engine RPM. Fit a taller tire or a lower-numbered axle and that RPM drops; a 3.90 final drive would push it to roughly 2,570 RPM.

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Tire upsize

Speed at 2,500 RPM, stock vs taller tire

Gear ratio 0.70 Final drive 3.55 Tire size 265/70R17 vs 225/45R17 RPM 2,500

At 2,500 RPM the 225/45R17 (24.97 in) does about 75 mph in this gear; swapping to a 265/70R17 (31.6 in) raises that to roughly 95 mph for the same RPM because each wheel turn covers more ground. The trade-off is slower acceleration and an optimistic speedometer until it is recalibrated.

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Deep Dive

Understanding the Gear Ratio Calculator

3 chapters · 4 min read

A gear ratio is one of the most useful numbers in any drivetrain: it decides how engine speed becomes road speed and how torque is multiplied on the way to the wheels. This calculator finds the ratio from tooth counts, then combines it with your final drive and tire size to give real-world speed and RPM figures — and a per-gear table for the whole transmission.

How gearing trades torque for speed

A gear ratio is simply the driven gear's teeth divided by the drive gear's teeth. A ratio above 1 is a reduction: the output turns slower than the input but with proportionally more torque — great for acceleration, climbing, and towing. A ratio below 1 is an overdrive: the output spins faster than the input, trading torque for speed and lower engine RPM. Every gear in a car multiplies through to the wheels as transmission ratio × final drive ratio, so first gear in a 3.36:1 ratio behind a 3.55 axle is a hefty 11.9:1 total reduction, while an 0.70:1 overdrive top gear is only 2.49:1 overall.

Why tire size and final drive matter

The transmission and axle set the ratio, but the tire sets the distance covered per wheel revolution. A larger overall diameter means more ground per turn, so speed per RPM rises and indicated RPM at a given speed falls. That is why an off-road tire upsize makes a truck feel lazier and throws the speedometer off until it is recalibrated. P-metric sizes like 225/45R17 encode everything you need: width 225 mm, sidewall 45% of width, and a 17-inch rim, which the calculator converts to an overall diameter and circumference automatically. You can also type a flotation size like 33X12.50R15 or just an overall diameter in inches.

Limits and edge cases

The speed figures are theoretical: they assume the engine can actually reach the chosen RPM in that gear, ignoring aerodynamic drag, rolling resistance, traction, and tire growth at speed. In a tall top gear the RPM required for a high speed can exceed redline, in which case that speed is simply unreachable in that gear — the calculator flags this. Real top speed is usually drag-limited well below the geometric maximum. Treat these numbers as a clean baseline for comparing gears, axles, and tire sizes, not as a guarantee of measured performance.

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Questions

Frequently Asked Questions

6 questions

Does a higher gear ratio number mean more torque or more speed?+

A numerically higher ratio (e.g. 4.10:1 vs 3.55:1) means more torque multiplication and quicker acceleration, but lower top speed and higher cruising RPM. A lower number means the opposite — taller gearing for relaxed, economical highway running. "Higher gear" in everyday speech (overdrive) is confusingly a lower number.

Do bigger tires change my effective gear ratio?+

Effectively, yes. A taller tire covers more ground per revolution, which acts like a taller (lower-numbered) final drive: more speed per RPM, slower acceleration, and a speedometer that reads low until recalibrated. Many people offset a tire upsize by fitting a numerically higher axle ratio to restore the original feel.

What is a good highway cruising RPM?+

Most modern cars cruise at roughly 1,800–2,500 RPM at 70 mph in top gear — low enough for good economy and quiet, high enough to stay responsive. Sports cars and trucks towing heavy loads often run higher. Use the Speed ↔ RPM tab in "RPM at speed" mode to check yours, then experiment with tire and axle changes.

How do I read a tire size like 225/45R17?+

225 is the section width in millimetres, 45 is the aspect ratio (sidewall height as a percentage of width, so 225 × 0.45 ≈ 101 mm), R means radial construction, and 17 is the wheel rim diameter in inches. The calculator converts this to an overall diameter (≈24.97 in here) and circumference for the speed math. You can also enter a flotation size (33X12.50R15) or a plain diameter in inches.

What is the difference between transmission gear ratio and final drive?+

The transmission gear ratio changes with the gear you select; the final drive (axle or differential ratio) is fixed and applies in every gear. Total reduction at the wheels is the two multiplied together. On a motorcycle the "final drive" is the front/rear sprocket ratio of the chain. The calculator keeps them separate so you can change one without the other.

Can I share or save my results?+

Yes. The calculator encodes your inputs in the page URL, so copying the link (or using the Share button) lets a colleague open the exact same scenario. Your last inputs are also stored locally in your browser, and the CSV button downloads the full per-gear speed table for spreadsheets.

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