Warning: no stdin data received in 3s, proceeding without it. If piping from a slow command, redirect stdin explicitly: < /dev/null to skip, or wait longer. Looking at the trending questions, I'll pick two fresh topics not covered by existing slugs:

1. Questions 1 & 2: Gearbox specs explained (BMW 316d and Fiat Ducato users asking about their 6-speed internals)
2. Questions 20 & 21: PS vs HP vs kW confusion (BMW owners asking "Wie viel PS hat er")

title: "What Do Gearbox Specs Actually Mean?" slug: "manual-gearbox-specs-explained" description: "Gear ratios, torque ratings, oil capacities: here's what the internal specs of a 6-speed gearbox actually mean and how to find them for your car." date: "2026-04-16" category: "Guides" keywords: "gearbox specs, gear ratios explained, manual transmission specifications, gearbox oil type, final drive ratio, 6 speed gearbox" author: "TorqueBot Team"

When someone asks for the "internal specs" of their gearbox, they're usually trying to figure out one of a few things. Is it strong enough for a modification? What oil does it take? Why does the speedo read wrong after a tyre change? Or they're about to buy a replacement unit and want to make sure it's the right one.

Gearbox specs sound dry, but they tell you a lot about how a car was designed to behave.

Gear Ratios: What They Actually Do

Every gear has a ratio. First gear might be 3.82:1, which means the input shaft (coming from the engine) rotates 3.82 times for every one rotation of the output shaft (going to the wheels). The higher the number, the more mechanical advantage you get, and the slower but more forceful the output.

Lower first gear ratios mean better towing and low-speed grunt. Higher numbers in top gear mean the engine spins slower at highway speeds, which saves fuel and reduces wear.

When 4th gear has a ratio of exactly 1.0, that's called a "direct drive" gear. Engine and output are spinning at the same speed with no multiplication happening. Everything above 1:1 is an overdrive gear, designed to keep revs low.

The Final Drive Ratio

This one often gets overlooked. The final drive (or diff ratio) is a multiplier that applies to all gears. It sits in the differential and turns the gearbox's output into actual wheel rotation.

A final drive ratio of 3.73:1 combined with a 6th gear ratio of 0.70:1 gives an overall drive ratio of 2.61:1 at the wheels. That's how fast your wheels spin relative to the engine.

Change your tyre size and this whole calculation shifts. A taller tyre effectively lengthens your final drive, making acceleration lazier but improving fuel economy at the same speed. That's why some people re-gear after lifting a 4WD and fitting bigger rubber.

What the Torque Rating Tells You

Every gearbox has a maximum input torque rating. The BMW F31 316d, for example, runs the N47 diesel engine producing around 270 Nm of torque. The 6-speed manual in that car (typically a Getrag 6-speed unit) is rated to handle that comfortably with a safety margin built in.

If you're tuning the car and pushing the engine past its stock torque output, the gearbox rating becomes relevant. Push a gearbox 30-40% beyond its torque rating consistently and synchros start dying, followed by the gear teeth themselves.

The torque rating is usually listed in the gearbox's service data or can be found via the manufacturer's technical documentation.

Oil Type and Capacity

This is probably the spec people need most often. Use the wrong gear oil and you'll wear the box out prematurely or cause synchro damage.

Most 6-speed manual gearboxes take either a GL-4 or GL-5 rated gear oil, but the two aren't interchangeable. GL-5 contains sulphur-based additives that protect hypoid gears but can corrode yellow metals (brass, bronze) commonly used in synchro rings. Always check what your specific box requires.

Viscosity matters too. A BMW Getrag unit typically calls for a specific MTF (manual transmission fluid) like Shell Spirax S4 G or Castrol Syntrans Multivehicle 75W-90. The Fiat Ducato's 6-speed manual (a Tremec or B&B unit depending on year) usually takes a 75W-80 or 75W-90 GL-4 fluid.

Capacity is usually 1.5 to 2.5 litres for a passenger car gearbox. Vans and light commercial vehicles like the Ducato can take closer to 2.5 to 3 litres.

How to Find the Actual Specs for Your Car

The most reliable sources, in order of accuracy:

Workshop manual: The definitive source. Either buy a Haynes/Chilton for older cars or access a digital workshop manual through services like ALLDATA or Autodata. These list gear ratios, oil spec, capacity, torque ratings and more.

Manufacturer technical data: BMW publishes a lot of this through their TIS (Technical Information System). Fiat/Stellantis has their ePER and eLearn systems. Some require a subscription, but often mechanics at a dealership can look it up quickly.

Gearbox tag or plate: Most manual gearboxes have a stamped or engraved tag on the casing. This usually gives you the gearbox model number. Search that number and you'll find the spec sheet.

Online forums: For common cars like the BMW 3 Series or Fiat Ducato, owners clubs and forums have usually compiled this information. It's not as authoritative as a workshop manual but it's a good starting point.

When Gearbox Specs Really Matter

For most people driving a stock car, you just need to know the oil type and capacity. Change the oil every 60,000 to 80,000 km (40,000 to 50,000 miles), use the right fluid, and the box will outlast the rest of the car.

Specs become critical when:

• Replacing a gearbox and confirming the new unit matches your engine's torque output
• Fitting larger tyres and recalculating speedo error and effective drive ratios
• Planning a tune or power increase and checking what the gearbox can handle
• Diagnosing a fault and confirming shift feel against the factory design intent

A box that slips into neutral under load usually has worn synchros or gear teeth. A box that crunches on shifts typically has degraded synchro rings, often from incorrect oil or oil that was left too long. Knowing the design specs helps you figure out whether the box is failing normally or has been treated badly. Try TorqueBot Free | Download on iOS | Get it on Android