How Do Gear Ratios Work?
A gear ratio compares the size of two meshing gears by counting their teeth. If a small gear with 10 teeth drives a larger gear with 32 teeth, the ratio is 32:10, or 3.2:1. The output gear turns once for every 3.2 turns of the input.
That reduction does two things at once, and they're opposite sides of the same coin. The output shaft turns slower than the input, but it turns with more torque, roughly in the same proportion. A 3.2:1 reduction multiplies input torque by close to 3.2 times, minus a small loss to friction. That's why first gear, with the highest ratio in the box, gives the strongest pull away from a standstill but runs out of road speed quickly: you're trading speed for torque. Top gear does the opposite, closest to a 1:1 ratio (sometimes even below it, an "overdrive" ratio), sacrificing low-speed shove for a faster, more relaxed top end.
What Is a Final Drive Ratio, and How Does It Combine With the Gearbox?
The final drive is a second reduction, downstream of the gearbox, inside the differential. It's fixed, one number for the whole car rather than one per gear, and its job is the same as any gear ratio: trade speed for torque, permanently, at every gear.
To get the overall ratio for any given gear, multiply the gearbox ratio for that gear by the final drive ratio. Take a first gear of 3.2:1 and a final drive of 4.1:1:
3.2 x 4.1 = 13.12
That 13.12 is the overall reduction between the engine and the wheels in first gear: for every 13.12 turns of the engine, the wheel turns once. Do the same sum for every gear and you get the full spread the gearbox and diff produce together, typically a wide overall ratio in first for maximum pull, narrowing gear by gear down to a small overall ratio in top for relaxed cruising. Our gear ratio speed calculator runs this multiplication automatically for every gear in your box, alongside road speed and redline in each.
How Do You Work Out Road Speed From RPM and Tyre Size?
Once you have an overall ratio, the last piece is how far the car travels for each wheel turn, the tyre's rolling circumference.
Take a common 205/55 R16 tyre. The sidewall height is the tyre's width multiplied by its profile percentage: 205mm x 0.55 = 112.75mm. Add that twice to the rim diameter (16 inches x 25.4mm = 406.4mm) to get the overall tyre diameter:
406.4 + (2 x 112.75) = 631.9mm
Circumference is diameter x pi: 631.9mm x 3.14159 = 1,985mm, or about 1.98 metres per wheel revolution (rounded to the nearest centimetre).
Now put rpm, overall ratio and circumference together. At 3,000 engine rpm, in first gear (overall ratio 13.12 from the example above), the wheel turns at 3,000 divided by 13.12 = 228.7 revolutions per minute. Multiply by the circumference and by 60 to get metres per hour, then convert to mph:
228.7 rev/min x 1.985m = 453.9 m/min, which gives 27,236 m/hour, which gives 27.2 km/h, which gives 16.9 mph
So a car with that first gear, final drive and tyre size is doing roughly 17mph (rounded) at 3,000rpm in first, which is exactly why you change up well before redline in the lower gears. A separate question is what the wheel alone is doing: strip the gearbox and final drive out entirely and you're left with a straight rpm-to-speed conversion for the tyre on its own, which is what the wheel speed calculator is built for. Plug your own numbers into the gear ratio speed calculator to get every gear at once, including speed at redline and cruising rpm at any road speed.
How Does Gearing Affect Acceleration and Cruising?
Shorter (numerically higher) gearing, a lower first gear, a taller final drive, or smaller tyres, increases the torque multiplication at the wheels, so the car accelerates harder for the same engine output. The trade-off shows up at the other end: every gear also has a lower top speed and a busier cruise, because the engine has to spin faster to cover the same ground.
Taller (numerically lower) gearing does the reverse: less multiplication, so gentler acceleration, but a more relaxed, quieter, more economical cruise, because the engine turns fewer times per mile. Motorway-biased cars lean tall; track and towing setups lean short. For the weight limits that matter when setting up a tow car and trailer, read the towing capacity guide. There's no ratio that gives both a hard launch and a lazy cruise, shortening one end of the range always lengthens the trade at the other.
Why Don't Electric Cars Need Multiple Gears?
Petrol and diesel engines only produce useful torque across a narrow rev range, which is why they need several gears to keep the engine in that range as road speed changes. Electric motors don't have that problem: they produce close to maximum torque from a standstill and can spin efficiently across a far wider range, often to 15,000-20,000rpm.
That means a single, fixed reduction gear, functionally just a final drive with no gearbox in front of it, can cover everything from a standing start to motorway speed without hunting for the right ratio. It's the same maths as everywhere else on this page, just with only one overall ratio to work out instead of five or six. If you're weighing an EV against a petrol equivalent, our torque and power converter is useful for comparing the figures each side quotes. To put engine output against kerb weight for an acceleration estimate, try the 0-60 calculator.
Quick Answers
Is a "3.2 first gear" the same as a "3.2:1 ratio"? Yes, the ratio number on its own always means input turns per output turn, so "3.2 first" and "a 3.2:1 first gear ratio" describe the same thing.
Does a shorter final drive always mean faster acceleration? In isolation, yes, for a given gearbox and tyre size, more torque multiplication at every gear. In practice it also raises rpm and fuel use at any given road speed, so it's a trade, not a free upgrade.
Why do the gear ratios get closer together going up the box? Narrower gaps between the higher gears keep the engine closer to its powerband after each change, since the percentage speed increase needed to keep pulling matters more than the absolute ratio difference higher up.