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Gearhead Diffs

Diff ratio calculators

Four tools for working out what your gearing is doing. Figures are a guide based on geometric rolling diameter; real rolling radius varies with load and tyre pressure.

Diff ratio finderEngine RPMTyre size and speedo errorWork out my current ratio

Diff ratio finder

Tell us the revs you want at a cruising speed and we'll work out the ratio. Everything updates as you type.
Gearbox top gear ratio
Tyre diameter: 777 mm
How to use this one

Pick the revs you'd like to see on the tacho at your usual open road speed. Most diesels are happy around 1,800 to 2,100 rpm at 100 km/h; petrol V8s around 1,800 to 2,000 in overdrive. Choose your gearbox's top gear (a direct top gear is 1.00; most overdrives are 0.70 to 0.85) and the tyre size off the sidewall. The ideal ratio is the exact maths; the closest common ratio is what you can actually buy.

IDEAL RATIO 4.19 Closest common ratio: 4.10, which gives about 1,958 rpm at 100 km/h.
We stock 4.10 for these vehicles
Holden Colorado RG Rebuilt Exchange Diff 3.42 / 3.73 / 4.10 · in stock · $2,095 Holden Commodore VT to VZ M80 LSD Exchange Diff 3.08 / 3.73 · in stock · from $2,295 Nissan Navara D40 Rebuilt Exchange Diff 3.69 · in stock · $2,995 Toyota Land Cruiser 70 and 100 Series Rebuilt LSD 3.90 · in stock · $3,295 Toyota Land Cruiser 80 Series LSD Exchange Diff 4.10 · in stock · $2,395 Nissan Safari / Patrol H233B Front Exchange Diff 4.11 · in stock · $1,995 Ford Courier 2WD LSD Rear Exchange Diff 4.10 · in stock · $1,995 Mazda Bounty / Ford Courier 4WD LSD Rear Exchange Diff 4.44 · in stock · $3,295 Ford 9 Inch Ring and Pinion 3.25 to 4.10 · from $795
Around 120 rebuilt diffs on the shelf. If yours isn't here, ask. We may be able to source what you need. Ask us about a ratio change

Engine RPM calculator

Diff ratio, gearbox top gear, tyre size and speed in; cruising revs out. Handy for checking a ratio change before you commit.
ENGINE RPM 2,545 at 100 km/h on a 777 mm tyre turning 682 rpm
98 km/h at 2,500 rpm
How to use this one

Use 1.00 for a direct top gear or the overdrive figure from your gearbox spec (common: 0.70 to 0.85). Enter any gear's ratio to see revs in that gear instead. Tyre size as printed on the sidewall. The result is wheel rpm times diff ratio times gearbox ratio; a torque converter or slipping clutch will add a little on top.

Tyre size and speedo error

Compare two tyre sizes (metric, imperial or plain diameter) to see the speedo error and effective gearing change.
Diameter777 mm to 834 mm (+7.3%)
Speedo readsslow: at an indicated 100 you're doing 107.3 km/h
Effective ratio3.48 (feels like it)
Ratio to restore gearing4.00, closest common 3.90
How to use this one

Enter the size you came off and the size you're going to. Accepted: 265/70R16, 33x12.5R15, 31 (inches) or 800 (mm). A bigger tyre lowers the effective gearing and makes the speedo read under; a smaller one does the opposite. The ratio to restore gearing is current ratio times new diameter over old diameter. Pick the nearest ratio made for your axle, or ask. We may be able to source what you need.

Work out my current ratio

Jack it up, mark the tyre and tailshaft, count tailshaft turns per wheel turn. Or enter the tooth counts if the diff is apart.
Method
YOUR RATIO 3.75 Closest common ratio 3.73
Got the tooth count instead?
Ring ÷ pinion = 3.727:1
How to use this one

Gearbox in neutral, handbrake off, driven axle on stands. Chalk a mark on a tyre and one on the tailshaft. Turn the wheel one full turn and count the tailshaft turns, including the fraction. With an open diff and one wheel locked, the free wheel has to turn twice for the same tailshaft count, so pick that method and we halve it for you. Turning the wheel 2 or 3 times and entering those counts gives a more accurate answer. Not sure which axle you have? Read the diff identification guide.

Common questions

How do I work out my diff ratio?

Divide the number of teeth on the crownwheel (ring gear) by the number of teeth on the pinion. A 41 tooth crownwheel on a 10 tooth pinion is 4.10:1. With the diff still in the vehicle, jack both rear wheels up, mark the tailshaft and a tyre, turn the wheels one full turn and count the tailshaft turns. That count is the ratio.

What engine RPM will I have at 100 km/h?

Engine RPM = wheel RPM × diff ratio × gearbox ratio. Wheel RPM is road speed in metres per minute divided by the tyre circumference in metres. A 265/70R16 tyre is about 777 mm in diameter, so at 100 km/h it turns about 680 rpm. With a 3.73 diff and a 0.70 overdrive top gear that is roughly 1,780 rpm.

What happens to my gearing if I fit bigger tyres?

Bigger tyres lower the effective gearing and make the speedo read slow. Going from 265/70R16 to 285/75R16 is about 7 percent more diameter, so the engine turns about 7 percent fewer revs at the same speed and the speedo reads about 7 percent under. To get the original gearing back, multiply the current diff ratio by the new tyre diameter divided by the old one.

Do you stock the ratio the calculator suggests?

We keep around 120 rebuilt exchange diffs on the shelf in the common ratios for popular utes, 4WDs and Commodores and Falcons, plus ring and pinion sets for the Ford 9 inch and others. If the ratio you need is not listed, ask. We may be able to source what you need.

Found the ratio you need?

Send us your rego and the ratio and we'll tell you what's on the shelf. Same working day reply.
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