Torque and gear ratio: the physics of car motion

When it comes to car performance, most drivers first pay attention to horsepower and acceleration to 100 km/h. However, these numbers hide two fundamental concepts that directly determine how a car behaves on the road: torque and gear ratio. Why does one engine “pull” from the bottom, while the other spins up only at high speeds? Why do trucks do without 8-speed transmissions, but sports cars need short gears? The answers lie precisely in these physical quantities.

In this article we will look at what torque is and how it relates to power, why gear ratios in the gearbox and final drive are so important for dynamics, and how to choose them correctly for different operating conditions - from city traffic jams to race tracks. And let's find out why Porsche 911 could be faster with rear wheel drive Audi RS6 with all-wheel drive despite less power, and how that relates to gear ratios.

What is torque and why is it more important than horsepower?

Torque is the force with which the engine crankshaft rotates, measured in newton meters (Nm). To simplify, this is the same “thrust” that you feel when the car takes off sharply or accelerates confidently at low speeds. Power (horsepower) is a derivative of torque and engine speed. The connection formula is simple:

Power (hp) = (Torque × RPM) / 7023

This means that the engine can produce the same power at different combinations of torque and rpm. For example:

• 🔧 Diesel engine: high torque (500 Nm) at low speeds (1500 rpm) → power ~105 hp
• ⚡ Gasoline aspirated: low torque (200 Nm) at high speeds (6000 rpm) → power ~171 hp.

This is why diesels are so good for trucks and SUVs: they pull from the bottom without requiring constant gear changes. Sports gasoline engines, on the contrary, reveal their potential only at high speeds, but they allow you to reach a higher maximum speed.

Gear ratio: how a gearbox “transforms” torque into speed

The gear ratio is the ratio of the number of teeth on the driven gear to the number of teeth on the drive gear. In the context of a car, it shows how many times does the gearbox or main gear increase (or decrease) the torque?, transmitted to the wheels. For example:

• 🚗 First gear: gear ratio 3.5–4.0 → torque increases 3.5–4 times, but wheel speed decreases.
• 🏁 Fifth/sixth gear: gear ratio 0.7–0.9 → the torque almost does not increase, but the wheels rotate faster.

Simply put, short passes (with a large number) give powerful acceleration, but a low maximum speed, and long (with a small number) - vice versa. For example, in Toyota Supra MK4 first gear has a ratio of 3.166, and sixth - only 0.759. This allows the car to take off sharply and at the same time accelerate to 250+ km/h.

The main pair (differential) also has its own gear ratio. U BMW M3 (E46) it is equal to 3.62, and Nissan GT-R R35 - 3.70. This means that GT-R will accelerate faster at low speeds, but its top speed will be lower at the same engine speed.

How to choose gear ratios for tuning

If you're planning on modifying your car, choosing the right gear ratios can dramatically change its character. Here are the key rules:

1. For drag racing (straight-line acceleration): Shorten first and second gears (increase the gear ratio). For example, in Honda Civic Type R Tuners often install a 4.7 main pair instead of the standard 4.1.
2. For track racing: Optimize gears for a specific track. On Nürburgring High speed on the straights is important, so long passes are preferable.
3. Off-road: increase the gear ratio in the transfer case (for example, from 2.48 to 4.10 in Jeep Wrangler). This will give more traction at low speeds.

However, there are nuances:

⚠️ Attention: Shorter gears will increase the load on the engine and transmission. If the engine is not prepared (connecting rods, crankshaft, clutch are strengthened), the risk of breakdown increases significantly. For example, standard clutch VW Golf GTI cannot withstand constant starts with a first gear ratio below 3.0.

Make sure the engine can handle the increased load|Check transmission and differential life|Calculate the new maximum speed in each gear|Consider the change in fuel consumption (shorter gears increase it by 10-20%)|Test drive in a closed area-->

Table: Gear ratios of popular cars

For clarity, let’s compare the gear ratios of gearboxes and main pairs in different cars. Notice how they affect the dynamics:

Model	Gearbox type	1st gear	5th/6th gear	Main couple	Acceleration 0–100 km/h (sec)
Toyota GR Supra (2020)	Automatic (ZF 8HP)	4.714	0.667	3.15	4.3
Ford Mustang GT (2018)	Mechanics (Tremec TR-3160)	3.66	0.63 (6th)	3.55	4.0
Nissan GT-R R35	Robot (GR6)	3.827	0.775 (6th)	3.70	2.9
Jeep Wrangler Rubicon	Mechanical/Automatic	4.46 (4:1 transfer case)	0.79 (5th)	4.10	10.5

Key takeaway: The Nissan GT-R has the shortest first gear ratio among the cars presented (3.827), which explains its record-breaking acceleration. And the Jeep Wrangler, with its 4:1 transfer case ratio and 4.10 final drive, is optimized for off-road use, not speed.

How to calculate the gear ratio yourself

If you want to know exactly what gear ratios your car has, or plan to change them, you will need:

1. Number of teeth on the drive and driven gears (for a manual transmission).
2. Gearbox marking (usually indicated on the housing).
3. Data on the main pair (can be found in the manual or by VIN code).

Calculation formula:

Gear ratio = Number of driven gear teeth / Number of drive gear teeth

Example: at a checkpoint VAZ 2110 first gear has 37 teeth on the driven gear and 12 on the drive gear. This means the gear ratio = 37 / 12 ≈ 3.08.

For automatic boxes (eg ZF 8HP or Aisin TF-80SC) gear ratios are fixed and are indicated in the technical documentation. They can only be changed by replacing the planetary gears, which is extremely expensive and difficult.

How to find out gear ratios by VIN code?

Many manufacturers (eg. BMW, Audi, Toyota) encode transmission information into the VIN. Use services like vin-decoder.net or official dealer databases. For example, in VIN BMW M5 (F90) code 2MK stands for box ZF 8HP75 with the main pair 3.15.

The influence of gear ratios on fuel consumption

Many people forget that gear ratios directly affect fuel consumption. The “longer” the gears (smaller gear ratio), the lower the engine speed at the same speed, which means less consumption. For example:

• 🚘 Skoda Octavia 1.6 TDI with a 5MT gearbox and a 3.45 main gearbox it consumes 4.2 l/100 km on the highway.
• 🚗 The same car, but with the main pair 4.10 (shortened) - already 4.8 l/100 km.

However, there is a downside: too long gears force the driver to shift more often in the city, which also increases consumption. The optimal balance depends on your riding style:

• 🏙️ City: gear ratios 3.5–4.0 for first gear, main gear 3.7–4.3.
• 🛣️ Route: first gear 3.0–3.5, main gear 3.0–3.5.

⚠️ Attention: If you install a shortened main pair (for example, 4.5 instead of 3.9), but drive mostly on the highway, the engine will constantly run at high speeds. This will lead not only to increased consumption, but also to accelerated wear of the piston group and turbine (if there is one).

Common mistakes when choosing gear ratios

Even experienced tuners sometimes make mistakes when selecting gear ratios. Here are the most common mistakes:

1. Ignoring maximum power speed. If the engine produces peak power at 6500 rpm, and you set the main gear to 4.5, then in 6th gear at 120 km/h the rpm will be over 4000 - far from optimal. Result: loss of dynamics and excessive fuel consumption.
2. Not taking into account the weight of the car. Short gears on a heavy machine (for example, Mercedes S-Class) will lead to constant shifting and overheating of the transmission.
3. Neglect of the transfer case (for all-wheel drive cars). B Subaru Impreza WRX STI transfer ratio 1.1:1, and in Mitsubishi Lancer Evolution — 1.03:1. This affects the distribution of torque between the axles and requires an integrated approach when tuning.

To avoid mistakes, use online gear ratio calculators (eg. gearcalc.com) or consult a transmission specialist.

FAQ: Answers to frequently asked questions

Is it possible to change gear ratios in a CVT?

In the classical sense, no. A variator (CVT) does not have fixed gears, but uses a belt and cones to smoothly change the gear ratio. However, some manufacturers (for example, Subaru in the model WRX CVT) offer “virtual gears” with fixed points that simulate a stepped gearbox. Real tuning of the CVT requires replacing the belt, pulleys and flashing the ECU, which is extremely expensive and not always effective.

How can I find out what gear ratios my car has?

Methods:

1. Look in the technical documentation (manual, manufacturer's website).
2. Find the markings on the gearbox or final drive housing (for example, GL5 3.94 - main pair with gear ratio 3.94).
3. Use diagnostic equipment (eg Launch X431) to read data from the ECU.
4. Calculate empirically: record the engine speed at a certain speed (for example, 60 km/h in 4th gear) and compare with the tables for your model.

Is it true that diesel engines don't need short gears?

Partly yes. Diesels have high torque at low speeds (e.g. BMW B57 produces 620 Nm already at 1500 rpm), so they do not require such “short” gears as gasoline engines. However, for sports diesels (for example, Audi S5 TDI) sometimes use shortened main pairs (3.8–4.0) to improve overclocking.

What is a “transfer case gear ratio” and what does it affect?

The transfer case (in all-wheel drive and off-road cars) has its own gear ratio, which determines how many times the torque increases when downshifting is engaged. For example:

• Toyota Land Cruiser 200: transfer case with gear ratio 2.6 (bottom row) and 1.0 (top row).
• Jeep Wrangler Rubicon: transfer case 4:1 (one of the shortest for production cars).

This number directly affects cross-country ability: the larger it is, the easier it is for the car to overcome off-road conditions, but the lower the maximum speed in low gear. For example, on Wrangler Rubicon with a 4:1 transfer case and a 4.10 main gear in first gear, the total gear ratio will be 4 × 4.10 × 3.8 (gearbox) ≈ 62.9. This means that the torque increases by 62.9 times, but the speed is limited to ~5 km/h at 3000 rpm.

Is it possible to drive with a faulty main gear?

Strongly not recommended. Symptoms of main pair malfunction:

• 🔊 A hum or howl from the differential that intensifies during acceleration.
• 🛑 Vibrations or jerks when coasting.
• 💨 Oil leak from the rear axle gearbox.

Driving with a damaged main pair leads to:

1. Accelerated wear of bearings and differential pinions.
2. Risk of wheel jamming while driving (especially dangerous at high speed).
3. Damage to axle shafts and CV joints.

The average cost of repairing the main pair (for example, on Ford Focus ST) - from 20,000 to 50,000 rubles, depending on the model and degree of wear.