Gear Ratio Calculator Kilometres per Hour: Formula, Worked Example & RPM Tables

Gear Ratio Calculator Kilometres per Hour comes down to one formula and three inputs: the road speed, the axle ratio and the tyre diameter. The formula is RPM = (speed × gear ratio × final drive × 336) ÷ tyre diameter in inches, and this page applies it to the question in the keyword with a worked example, tables at 30, 50 and 70 mph, and the practical effects explained in plain terms. The tables assume 28-inch tyres (example diameter) and a 1:1 top gear unless stated otherwise, and the assumptions are printed with each table so the numbers can be reworked for a different tyre size or an overdrive gear. Everything here is a calculation from stated inputs, and the disclaimer below applies to all of it.

For gear ratio calculator kilometres per hour, the point is this: The reason the ratio matters is that it sets the engine speed at every road speed, and engine speed decides how the vehicle feels: how hard it pulls away, how it holds speed on a grade, how busy the motorway cruise is, and how much fuel a steady run uses. Change the ratio or the tyre diameter and the whole speed-versus-RPM relationship shifts, which is why the two are always considered together. The worked example below shows the arithmetic step by step, the comparison table puts the common ratios side by side, and the later sections cover the mistakes that waste money and the checks that should happen before any parts are ordered.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

How the gear ratio formula works for gear ratio calculator kilometres per hour

The arithmetic is straightforward once the four inputs are clear. RPM = (speed in mph × gearbox gear ratio × axle ratio × 336) ÷ tyre diameter in inches. The 336 folds together the 5,280 feet in a mile, the 12 inches in a foot and the 60 minutes in an hour, so it is a constant of the units rather than a property of the vehicle. If the gearbox is in a 1:1 gear, the gear ratio is 1.00 and the formula shortens to RPM = (speed × axle ratio × 336) ÷ tyre diameter. Rearranged, speed = (RPM × tyre diameter) ÷ (gear ratio × axle ratio × 336), which answers the opposite question: how fast is the vehicle going at a given engine speed. Every table in this article is built from those two formulas with the assumptions stated next to it, so any line can be re-checked. That is the point this page works through for gear ratio calculator kilometres per hour.

gear ratio calculator kilometres per hour: What a gear ratio is

On this page, gear ratio calculator kilometres per hour is the case in point: A gear ratio describes how many times the driveshaft turns for each turn of the wheels. A 3.73 axle ratio means the driveshaft rotates 3.73 times while each wheel rotates once, and a 4.10 ratio means the driveshaft rotates 4.10 times for that same single wheel rotation. The number is fixed by the ring gear and pinion inside the differential: divide the number of teeth on the ring gear by the number on the pinion and you get the ratio. A higher number is called a shorter or lower gear, because it multiplies engine torque more and keeps the engine spinning faster at any road speed; a lower number is called a taller or higher gear, because it does the opposite. Neither is better in the abstract — the right ratio depends on the tyre diameter, the weight being moved, the engine's power band and how the vehicle is used.

gear ratio calculator kilometres per hour: Worked example: RPM at 30, 50 and 70 mph

On this page, gear ratio calculator kilometres per hour is the case in point: Take a 3.73 axle ratio on 28-inch tyres (example diameter), in top gear at 1.00. At 70 mph the formula gives RPM = (70 × 1.00 × 3.73 × 336) ÷ 28 = 3,133 RPM. At 50 mph it gives 2,238 RPM and at 30 mph 1,343 RPM. The speed at 2,000 RPM is 44.7 mph and at 3,000 RPM it is 67.0 mph. With a 3.07 ratio the 70 mph figure falls to 2,579 RPM, and with a 4.10 it rises to 3,444 RPM — the full spread is in the table below, and every line of it comes from the same formula with the same stated assumption.

gear ratio calculator kilometres per hour: RPM table: 3.07, 3.55 and 4.10 at 30, 50 and 70 mph

Assumption: 28-inch tyres (example diameter), top gear 1.00. RPM = (speed × ratio × 336) ÷ 28. That is the point this page works through for gear ratio calculator kilometres per hour.

Speed 3.07 ratio 3.55 ratio 3.73 ratio 4.10 ratio
30 mph 1,105 RPM 1,278 RPM 1,343 RPM 1,476 RPM
50 mph 1,842 RPM 2,130 RPM 2,238 RPM 2,460 RPM
70 mph 2,579 RPM 2,982 RPM 3,133 RPM 3,444 RPM

Taking gear ratio calculator kilometres per hour as the case in point: The table makes the trade concrete. At 70 mph the 3.07 cruises at the lowest RPM and the 4.10 at the highest, with a gap of about a third between them, and the same gap appears at 30 and 50 mph because the ratio multiplies the RPM at every speed. A taller ratio suits a vehicle that spends its life cruising; a shorter ratio suits one that tows, climbs or runs bigger tyres. The middle ratios are the compromise, and the worked example above shows how to rework any line of the table for a different tyre diameter: divide the table's RPM by the old diameter and multiply by the new one, or run the formula again from the start.

gear ratio calculator kilometres per hour: How tyre diameter changes the answer

For gear ratio calculator kilometres per hour, the point is this: Diameter matters because the formula divides by it. Double the tyre diameter and the RPM at a given speed halves; increase it by 10% and the RPM falls by roughly 9%. That relationship is also why the speedometer changes with tyre size: the speedometer assumes a fixed number of wheel revolutions per mile, and a bigger tyre covers the mile in fewer revolutions. When this article quotes a tyre diameter it is the nominal new-tyre figure from the size code — for example 205/55R16 measures 631.9 mm, or 24.88 inches — and real tyres vary slightly by brand, model, wear and inflation pressure, so the RPM numbers that follow are good to within a small margin rather than exact.

What the ratio does on the road for gear ratio calculator kilometres per hour

On this page, gear ratio calculator kilometres per hour is the case in point: The practical effects of a ratio are felt in three places: acceleration, cruising and towing. A numerically higher ratio multiplies torque more, so the vehicle pulls harder from rest and holds speed better with a trailer, and the engine reaches its useful RPM sooner after each shift. The same ratio raises cruising RPM, which means more noise and usually more fuel on a long steady run. A numerically lower ratio reverses all of that: calmer cruising and lower fuel use at steady speed, but softer acceleration and more frequent downshifts when the road rises or the load grows. Tyre size moves the whole picture — bigger tyres make any ratio act taller — so the right comparison is always ratio and tyre diameter together, never the ratio on its own. That is the comparison the tables in this article make.

How to find the ratio already fitted for gear ratio calculator kilometres per hour

Taking gear ratio calculator kilometres per hour as the case in point: The ratio on the vehicle can be found in several ways, and checking two of them is worth the effort. Start with the metal tag on the differential cover, which usually carries the ratio as a stamped number. Then check the door-jamb placard and the owner's manual, which list the specification as the vehicle left the factory. The VIN can be run through a decoder or the manufacturer's parts system, which returns the build specification including the axle — useful, but it describes the vehicle as built, not as modified, so a swapped axle will not show up. The definitive check is the tooth count: with the differential cover off, count the teeth on the ring gear and on the pinion and divide the first by the second. A 41-tooth ring gear with an 11-tooth pinion is 41 ÷ 11 = 3.73. When the sources disagree, the physical count wins.

Common mistakes with gear ratios for gear ratio calculator kilometres per hour

Taking gear ratio calculator kilometres per hour as the case in point: Mistakes usually come from using half the formula. The first is to pick a ratio from a chart without entering the actual tyre diameter — the chart's assumption may be a different size from the tyres on the vehicle, and the answer follows the assumption, not the vehicle. The second is to forget that the gearbox gear matters: first gear multiplies torque far more than top gear, so a crawl or launch question needs the first-gear ratio while a cruising question needs top gear. The third is to read a higher ratio as simply better: it raises cruising RPM, which can raise noise and fuel use on long motorway runs, so the gain in acceleration is paid for at cruising speed. The fourth is to trust a ratio stamped on an axle tag without checking that the axle is original to the vehicle — a swapped axle carries its own ratio. And the fifth is to treat the calculation as permission: the maths says what the RPM will be, not that the change is suitable, so a drivetrain specialist should confirm the choice before parts are ordered.

gear ratio calculator kilometres per hour: Related pages

Frequently asked questions

How do I work out the RPM for gear ratio calculator kilometres per hour?

For gear ratio calculator kilometres per hour, the point is this: Use RPM = (speed × gear ratio × final drive × 336) ÷ tyre diameter in inches, with the gear in use and the axle ratio for the vehicle. The worked example above shows the arithmetic at 30, 50 and 70 mph with the assumptions stated, and the gear ratio calculator runs the same formula with your own numbers.

What does a higher gear ratio do?

For gear ratio calculator kilometres per hour, the point is this: A numerically higher ratio — a shorter gear — raises engine speed at every road speed, multiplies torque more at the wheels, sharpens acceleration and helps towing, at the cost of higher cruising RPM and usually more fuel on steady motorway runs. A lower number does the opposite. The table above shows the RPM for several ratios side by side so the trade is visible.

How does tyre size affect gear ratio?

Taking gear ratio calculator kilometres per hour as the case in point: A larger tyre diameter makes the effective gearing taller: the vehicle travels further per wheel revolution, so the engine turns more slowly at a given road speed. A 10% larger diameter cuts cruising RPM by roughly 9% with the same ratio. That is why bigger tyres are usually paired with a numerically higher ratio, to restore the RPM the vehicle had before the tyre change.

Does changing the gear ratio affect the speedometer?

Yes, indirectly: the speedometer is calibrated for a rolling circumference, and a ratio change does not change the tyre, so the speedometer reading itself is unaffected by the ratio alone. What changes is the engine speed at a given road speed. A tyre-size change does affect the speedometer, because it changes the rolling circumference — see the speedometer error calculator for that calculation. That is the point this page works through for gear ratio calculator kilometres per hour.

Can I change my gear ratio myself?

On this page, gear ratio calculator kilometres per hour is the case in point: A ratio change means replacing the ring gear and pinion and setting the backlash and preload precisely, which is a specialist drivetrain job with special tools. An incorrect setup fails quickly and expensively. Most owners have the work done by a drivetrain specialist, who can also confirm the ratio suits the axle, the gearbox and the way the vehicle is used.