Is it better to have a bigger or smaller rear axle speed ratio?

 When buying a truck, you should consider the rear axle speed ratio and other factors according to the actual needs in order to choose the most suitable model for you.

Rear axle speed ratio is just a common name, the academic name is main reduction ratio, it is the gear ratio of the main reducer in the drive axle of the vehicle, attenuating the speed on the driveshaft, which can act as a deceleration to increase the torque. For example, if a truck has a rear axle speed ratio of 3.727, then if the driveshaft speed is 3.727 r/s, it will be attenuated to 1 r/s.

When talking about the effect of rear axle speed ratios on a vehicle, it is important to make sure that the vehicles being compared are the same model, as simply comparing the size of the rear axle speed ratios is of no practical significance if different models are involved and can easily lead to erroneous conclusions being drawn.

Because the rear axle is used in conjunction with the transmission, the gear ratios are different for different transmissions.

Why are trucks with smaller rear axle speed ratios faster?

Disregarding external factors such as load, wind resistance, uphill resistance, etc., and considering only the gear ratios, we can derive the vehicle speed by a formula:

Vehicle speed = 0.377 x (engine output rpm x tire rolling radius)/(gear ratio by transmission x rear axle speed ratio) where 0.377 is a fixed factor.

Let's calculate the vehicle speeds for light-duty trucks A and B using the given parameters. Both trucks have 6 forward gears and 1 reverse gear, with the highest gear being an overdrive gear. The gear ratio is 0.78, and the rear axle ratios are 3.727 for truck A and 4.33 for truck B. Assuming the transmission is in the highest gear and the engine speed is 2000 RPM, with a tire rolling radius of approximately 0.3822 meters (for 7.50R16 tires).

For light-duty truck A:

Vehicle Speed = 0.377 × (2000 RPM × 0.3822 m) / (0.78 × 3.727) = 7.83 m/s (or approximately 28.2 km/h)

For light-duty truck B:

Vehicle Speed = 0.377 × (2000 RPM × 0.3822 m) / (0.78 × 4.33) = 7.12 m/s (or approximately 25.6 km/h)

Therefore, when the transmission is in the highest gear and the engine speed is 2000 RPM, light-duty truck A would have a speed of approximately 28.2 km/h, while light-duty truck B would have a speed of approximately 25.6 km/h.

Light truck A speed = 0.377 x (2000 x 0.3822) / (0.78 x 3.727) = 99.13 (km/h);

Light truck B speed = 0.377 x (2000 x 0.3822)/(0.78 x 4.33) = 85.33 (km/h);

In the same model, when the engine speed is 2000rpm, it is theoretically deduced that the speed of the light truck A with a small rear axle speed ratio reaches 99.13km/h, and the speed of the light truck B with a large rear axle speed ratio reaches 85.33km/h. Therefore, the vehicle with a small rear axle speed ratio runs faster and saves more fuel.

2. Why do trucks with large rear axle ratios go hills more easily?

The climbing ability of a vehicle is related to the drive force of the truck. The theoretical formula for truck driving force is:

Driving force = (engine output torque × gear speed ratio × main gear ratio × mechanical transmission efficiency) / wheel radius

Continue to use the above light truck A and light truck B to give an example, if the light truck A and light truck B respectively in the first gear, the engine output torque of 450 Nm, we calculate the light truck A and light truck B at this time to obtain the driving force:

Light truck A driving force = (450x6.32X3.72X0.98)/0.3937 = 26384.55 (Nm)

Light truck B drive force = (450x6.32X4.33X0.98)/0.3937 = 30653.36 (Newtons)

When the engine is in 1st gear and the engine output torque is 450 Nm, the driving force obtained by Light Truck A is 26,384.55 Nm, and the driving force obtained by Light Truck B is 30,653.36 Nm, so it is obvious that Light Truck B, which has a large rear axle ratio, obtains a larger driving force, and naturally, it is more powerful in climbing the hill.

Matching a high-horsepower engine with a lower rear axle ratio is a common practice and suitable for various scenarios. Vehicles equipped with high-horsepower engines, coupled with a lower rear axle ratio and an overdrive gear in the transmission, can achieve speeds of over 90 miles per hour without requiring the engine to run at very high RPMs.

And high-horsepower large speed ratio rear axle, fuel consumption is very impressive, suitable for dump trucks, cement mixer trucks, and some special working conditions.

So when we buy a truck we still have to choose based on our usage.

The optimal gear ratio is advantageous for the engine to operate at its optimal working condition and can also minimize wear on the gear transmission.