How it's calculated
Your wheels turn slower than the engine by the overall ratio (gear × final drive). Multiply wheel revs by the tire's circumference and you get road speed:
speed = (rpm ÷ (gear × final)) × π × tireDiameter
That's why a numerically lower final drive (e.g. 3.5 vs 3.9) gives higher top speed but lazier acceleration, and a taller tire raises your speed at any given RPM.
Gear ratio speed chart
Every ratio worth having, with the cruise revs each one lands you on. Generated by running the calculator above, so a row here cannot disagree with the tool.
Speed per 1,000 rpm, and motorway cruise revs
On a 225/45 R17 (634 mm) tyre. Overall ratio is your gear ratio × final drive — 5th at 0.85 behind a 3.90 diff is 3.32. Road speed is linear in engine speed, so the per-1,000 column scales to any rpm you like.
| Overall ratio | km/h per 1,000 rpm | mph per 1,000 rpm | rpm at 100 km/h | rpm at 70 mph |
|---|---|---|---|---|
| 2.00 | 59.8 | 37.1 | 1,670 | 1,880 |
| 2.25 | 53.1 | 33.0 | 1,880 | 2,120 |
| 2.50 | 47.8 | 29.7 | 2,090 | 2,360 |
| 2.73 | 43.8 | 27.2 | 2,280 | 2,570 |
| 3.00 | 39.9 | 24.8 | 2,510 | 2,820 |
| 3.15 | 38.0 | 23.6 | 2,630 | 2,960 |
| 3.27 | 36.6 | 22.7 | 2,730 | 3,080 |
| 3.42 | 35.0 | 21.7 | 2,860 | 3,220 |
| 3.55 | 33.7 | 20.9 | 2,970 | 3,340 |
| 3.73 | 32.1 | 19.9 | 3,120 | 3,510 |
| 3.90 | 30.7 | 19.0 | 3,260 | 3,670 |
| 4.10 | 29.2 | 18.1 | 3,420 | 3,860 |
| 4.30 | 27.8 | 17.3 | 3,600 | 4,050 |
| 4.56 | 26.2 | 16.3 | 3,820 | 4,300 |
| 4.88 | 24.5 | 15.2 | 4,080 | 4,600 |
| 5.13 | 23.3 | 14.5 | 4,290 | 4,830 |
| 5.38 | 22.2 | 13.8 | 4,500 | 5,070 |
| 6.00 | 19.9 | 12.4 | 5,030 | 5,660 |
| 7.00 | 17.1 | 10.6 | 5,850 | 6,590 |
| 8.00 | 14.9 | 9.3 | 6,710 | 7,560 |
| 10.00 | 12.0 | 7.4 | 8,330 | 9,390 |
| 12.00 | 10.0 | 6.2 | 10,000 | 11,270 |
| 14.00 | 8.5 | 5.3 | 11,760 | 13,250 |
What a final drive change actually costs you
Regearing is the cheapest way to make a car feel faster without making any more power, and the trade is always the same: acceleration and cruise revs move in opposite directions.
Go numerically higher — 3.90 to 4.30 — and every gear gets shorter. The car pulls harder, the engine spends more time in its power band, and 0–60 improves. It also revs higher at every road speed, so motorway cruising gets louder and thirstier, and you may run out of gear before you run out of road. Go numerically lower — 3.90 to 3.55 — and you get the opposite: relaxed cruising, better economy at a steady speed, and a car that feels lazier everywhere below it.
A worked example from the chart above: an overall ratio of 3.90 at 2,000 rpm on a 225/45 R17 is 61.3 km/h — 38.1 mph. Because speed is linear in engine speed, that one figure gives you every other: 4,000 rpm is 122.6 km/h, 6,000 rpm is 183.9 km/h.
The gear that decides how the car lives
First gear sets how hard it launches; top gear sets what you live with. Most of a road car's life is spent in its highest gear at a steady speed, so the overall ratio there decides your cruise rpm, your cabin noise and a meaningful slice of your fuel bill.
Rough targets for a petrol road car at motorway speed: 2,000–2,500 rpm is relaxed and economical, 2,500–3,000 is normal for a shorter-geared hot hatch, and over 3,500 means either a very short diff or a missing overdrive — fine on a track car, tiring on a commute. Diesels sit lower, typically 1,700–2,200.
The catch is that top-gear cruise and in-gear response are the same number. If you gear for a quiet motorway you also gear for a car that needs a downshift to overtake. There is no ratio that wins both — pick the one that matches how you actually drive.
Tyre size is a gear ratio too
The formula treats tyre diameter exactly like a ratio, because that is what it is. A taller tyre travels further per wheel revolution, so it raises your speed at any given rpm — the same effect as a numerically lower diff, without opening the axle.
It cuts both ways: going up a tyre size softens acceleration slightly and drops cruise revs, and going down does the reverse. That is also why a big tyre change breaks your speedometer, since the speedo assumes the factory circumference. Check the size change first with the tire size calculator — it gives the speedometer error directly.
Will regearing upset the speedometer?
It depends on where your car takes its speed signal. If the sensor sits on the gearbox output shaft — common on older cars — it counts propshaft revolutions and never sees the diff, so a final drive change puts the speedometer out by the full ratio change and needs recalibrating. If the signal comes from the ABS wheel sensors, as it does on nearly every modern car, the reading is taken after the diff and a final drive change makes no difference to it at all.
Tyre diameter changes affect both types, because neither sensor can see how big the tyre is.
FAQ
What gear ratio do I enter?
The individual gear you care about — e.g. 5th might be 0.80, 1st might be 3.5. Use 1.00 for a direct-drive top gear. Multiply it by your final drive to get the overall ratio used in the chart above.
Where do I find my final drive?
It's the differential/axle ratio, often on a sticker on the diff housing or gearbox, in the handbook, or decodable from the VIN or an axle code. Common road cars run 3.4–4.4; trucks and off-roaders go higher.
Is a higher or lower final drive better?
Neither — they are opposite compromises. Numerically higher (4.10 over 3.73) gives quicker acceleration and higher cruise revs; numerically lower gives relaxed cruising and lazier response. Track and drag cars go higher, long-distance cars go lower.
Does regearing hurt fuel economy?
At a steady cruise, yes — a shorter diff means more engine revolutions per mile. In mixed driving the effect is smaller than it looks, because a car that needs fewer downshifts and less throttle to keep up can claw some of it back. If your motorway cruise rpm goes up by more than about 500, expect to notice it.
Can I get the same effect from a smaller tyre instead?
Yes, and it is far cheaper. Dropping the overall diameter a few percent shortens the effective gearing everywhere. The limits are that you cannot change it much before the speedometer and the arch both complain, and it costs you contact patch and ride quality — a diff change has none of those side effects.
What is a "close ratio" gearbox?
One where the steps between gears are smaller, so the engine drops less rpm on each shift and stays nearer its power band. It makes a peaky engine much more usable and does nothing for a torquey one. The trade is that you cover the same speed range with less spread, so first is taller or top is shorter.