Does Driving at 80 or 90 km/h Save More Fuel on Highways?

With the introduction of new highways, some allowing a maximum speed of only 80 km/h for cars, a debate has emerged on whether increasing the speed limit to a minimum of 90 km/h would result in fuel savings.

Many argue that, at speeds above 80 km/h, cars operate less efficiently, leading to time wastage, increased fuel consumption, and driver frustration. Suggestions have been made to raise the speed limit to at least 90 km/h.

Many argue that, at speeds above 80 km/h

To shed light on this matter, Associate Professor Dr. Dam Hoang Phuc, Director of the Automotive Engineering Program at Hanoi University of Science and Technology, highlighted the crucial role of engine revolutions per minute (RPM) in fuel consumption. Different car models with varying technical specifications, such as engine type, cylinder capacity, and power, operate efficiently at different RPM ranges.

"Most gasoline engines operate optimally and economically between 1,200-1,600 RPM, while diesel engines have a broader and less stable range but generally stay below 2,000 RPM. This RPM, combined with the gearbox type in each vehicle, results in different fuel-efficient speed ranges," explained Dr. Phuc.

To save fuel, he advised drivers to maintain RPM at an optimal level, always below 2,000 RPM. Additionally, maintaining a stable speed, avoiding abrupt acceleration and braking, and regularly maintaining the vehicle according to the manufacturer's recommendations contribute to fuel efficiency.

In reality, despite having the same RPM and operating conditions, each vehicle achieves different speeds. For example, at a speed of 90 km/h and operating in the highest gear, one car may reach around 1,600 RPM, while others may exceed 2,500-3,000 RPM.

Larger speed limits will still need to adhere to standards and regulations

Experts assert that, for most current compact cars, driving at 90 km/h does not result in fuel savings compared to driving at 80 km/h. Moreover, as speed increases, the aerodynamic drag on the vehicle also increases. Research indicates that when the speed doubles, air resistance quadruples.

Experienced drivers suggest that maintaining speeds between 60-80 km/h is most fuel-efficient. However, it is necessary to increase speeds on highways to at least match those on national roads.

Automotive engineer Duong Trung Kien, director of a transportation company in Hanoi, noted that the optimal speed for compact cars is around 60-80 km/h, while trucks should maintain speeds of 50-70 km/h. However, using cruise control at a constant speed of 100 km/h, for example, does not necessarily consume more fuel.

Kien emphasized that driving at high speeds, around 90 km/h, may not save fuel as commonly believed. However, increasing speed limits on highways is essential, at least to match those on national roads.

Minister of Transport Nguyen Van Thang recently announced that the Ministry of Transport has been studying whether the speed standards on certain highways are realistic. This suggests that highways currently set at 80 km/h may be raised to 90 km/h. Larger speed limits will still need to adhere to standards and regulations. Therefore, by Q1 2024, maximum speeds on certain highways are expected to be raised from 80 to 90 km/h.