Gear Ratio Calculator
Get final drive from ring and pinion teeth, then overall ratios, speed at redline in every gear, cruising RPM, crawl ratio and the axle ratio for a tyre change.
Final drive
Gearbox ratios
Every gear
| Gear | Gearbox ratio | Overall ratio | mph at 6,500 rpm | RPM at 70 mph |
|---|---|---|---|---|
| 1st | 3.360 | 12.524 | 43.2 | 10,524 |
| 2nd | 1.910 | 7.119 | 76.1 | 5,982 |
| 3rd | 1.330 | 4.957 | 109.2 | 4,166 |
| 4th | 1.000 | 3.727 | 145.3 | 3,132 |
| 5th | 0.810 | 3.019 | 179.3 | 2,537 |
| 6th | 0.650 | 2.423 | 223.5 | 2,036 |
Overall ratio = gearbox ratio × transfer-case ratio × final drive. Road speed = RPM × tyre circumference × 60 ÷ (overall ratio × 63,360), the same thing as the RPM × diameter ÷ (ratio × 336) shortcut. Figures assume no tyre slip or torque-converter loss.
Changing tyre size
- Current tyre: 28.00 in diameter
- New tyre: 31.00 in diameter
- Rolling distance per turn changes by a factor of 1.1071
- 6th at 70 mph would drop from 2,036 rpm to 1,839 rpm
- Axle ratio needed to put it back: 4.13:1
Taller tyres lower engine RPM for the same road speed, which is why a lift kit usually calls for a numerically higher axle ratio to restore the original driveability.
Crawl ratio
1st (3.360) × transfer case (1.000) × final drive (3.727) = 12.52:1
At idle, roughly 5.32 mph.
Off-roaders usually quote crawl ratios from around 40:1 for a mild build up to 100:1 or more for a dedicated rock crawler. A transfer-case ratio of 1 means the figure is just first gear × final drive.
What is the Gear Ratio Calculator?
The ByteTools Gear Ratio Calculator starts where the driveline does: final drive is ring gear teeth divided by pinion teeth, so 41 over 11 is 3.
- Final drive from ring and pinion teeth, or entered directly as a ratio
- Editable gearbox ratio list with add and remove buttons, six speeds by default
- Overall ratio, speed at redline and cruising RPM for every gear in one table
- Crawl ratio from first gear × transfer case × final drive, plus idle speed
- Axle ratio needed to restore cruising RPM after a tyre-size change
- Tyre diameter in inches or millimetres; speeds in mph or km/h
How to use the Gear Ratio Calculator
- 1
Enter the ring and pinion tooth counts, or untick the box and type the axle ratio directly.
- 2
Set the transfer-case ratio — leave it at 1 if the vehicle has no transfer case.
- 3
Add, remove or edit the gearbox ratios in the list to match your gearbox.
- 4
Enter the tyre diameter, the redline, your cruising speed and the gear you cruise in.
- 5
Read the per-gear table, then enter a new tyre diameter to see the axle ratio that restores your cruising RPM.
About the Gear Ratio Calculator
The ByteTools Gear Ratio Calculator starts where the driveline does: final drive is ring gear teeth divided by pinion teeth, so 41 over 11 is 3.727:1. Multiply that by the transfer-case ratio and each gearbox ratio and you have the overall ratio for every gear, which is what actually determines road speed at a given engine RPM.
From there it builds the table people want — speed at redline in each gear, engine RPM at your cruising speed, and the crawl ratio of first gear × transfer case × final drive. It also answers the tyre question: fit a taller tyre and cruising RPM drops, so the tool works out the axle ratio you would need to put it back exactly where it was.
Road speed comes from RPM × tyre circumference × 60 ÷ (overall ratio × 63,360), the exact form of the familiar RPM × diameter ÷ (ratio × 336) shortcut. Everything runs in your browser with no uploads, and it assumes no tyre slip or torque-converter loss.
Frequently asked questions
How do you calculate a gear ratio from tooth counts?
Divide the driven gear's teeth by the driving gear's teeth. For a differential that is ring teeth ÷ pinion teeth, so a 41-tooth ring with an 11-tooth pinion gives 3.727:1 — the driveshaft turns 3.727 times for each turn of the wheels.
How do you work out speed from RPM and gear ratio?
Speed = RPM × tyre circumference × 60 ÷ (overall ratio × 63,360) for mph, where overall ratio is gearbox × transfer case × final drive. The shortcut MPH = RPM × tyre diameter ÷ (ratio × 336) is the same equation, because 63,360 ÷ (π × 60) is 336.14.
What axle ratio do I need after fitting bigger tires?
Multiply your current ratio by the new tyre diameter divided by the old one. Going from 28 inch to 31 inch tyres with a 3.73 axle needs 3.73 × 31 ÷ 28 = 4.13, so a 4.10 axle gets you very close to the original driveability.
What is a good crawl ratio?
Crawl ratio is first gear × transfer-case low × axle ratio. Around 40:1 suits a mild trail build, 70:1 to 80:1 is a common target for serious off-roading, and dedicated rock crawlers run well past 100:1. Anything under about 30:1 needs a lot of clutch slipping on steep ground.
Does a numerically higher axle ratio mean faster acceleration?
Yes, at the cost of higher cruising RPM and usually worse fuel economy. A 4.10 axle multiplies torque more than a 3.73, so the car accelerates harder, but the engine spins faster at any given road speed. The per-gear table here shows both effects at once.
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