Pressure saves fuel, doesn't it?
We have tested and demonstrated the dependency of fuel consumption on tire pressure.
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The dependence between fuel consumption and tire pressure has been known for a long time. Both professional and amateur drivers use the simple rule "if you want to save on fuel, pump up the wheels", but often they overpump it. Some people are ready to sacrifice both comfort and safety (bumps in the road are felt much more clearly on overpumped tires, the contact patch of the tire with the road decreases and braking distance increases). They forget that in pursuit of fuel economy there is a loss in tire service life. Sometimes it is possible to meet the opposite situation when the tires are not monitored and even visually look half flat. It is an extreme case, but drivers often don't know what pressure they have in the tires. Visually, a truck wheel inflated to 8 bar is indistinguishable from the 6 bar one. This difference in pressure we chose to conduct our experiment.

What is the point?
One night we met with representatives of the very famous network food company which agreed to provide its transport for the experiment on the City Ring Road. We decided to do our measurements at night in order to avoid traffic jams.

Two MAN TGS 4x2 trucks with the same three-axle semi-trailers were used for the experiment.

Of course, we understood perfectly well that there are no vehicles with exactly the same fuel consumption. Moreover, our trucks were already 4 years old. It is quite a mature age for commercial vehicles that affects fuel consumption as well. But we were not going to compare the fuel consumption between vehicles, we just needed to find how the tire pressure affects it.

We checked and equalized the tire pressure to 8 bars (in cold state), fully refueled the trucks and set up GalileoSKY GPS trackers and PressurePro TPMS.
truck gas mileage
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Both trucks are equipped with KAMA tires with 10% tread wear on the steering axle and around 50% on other wheels.
Drivers are ready to go as they got all the necessary instructions. 14 weighed and marked cans of diesel fuel are prepared to refuel the trucks upon return. Electronic scales with an accuracy of 5 grams will allow us to determine the volume of diesel in liters. The first stage is over, the MAN trucks are back.

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The first stage is over, the MAN trucks are back.
Both trucks covered 142 km with the average speed of 77 kmph but the fuel consumption was different.

The first one was refueled with 33.575 kg of fuel which is 39.5 liters, another one with 35.335 kg (41.57 liters). There is a definite difference.

The fuel consumption is 27.81 and 29.27 l/100 km, respectively.

We let the drivers to take some rest and the vehicles to cool down. Then we check the tire pressure again and make some adjustments.

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We decrease the tire pressure only on 4 wheels out of 12 in the driving axle of the first truck down to 6 bar.

These 4 wheels are visually indistinguishable from the others, which are still pumped to 8 bar.

The tire pressure of the second truck remains the same.
Everything is ready for the second lap.
We expect that the fuel consumption of both trucks will change due to the road and weather conditions, driving style, etc. But for the first truck the change will be caused by the decreased tire pressure in the driving axle as well.
Here are the results of the second stage.

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The mileage and average speed remained the same. But the first truck, which showed the best economy in the first stage, became an outsider this time.

The first truck was refueled with 36.428 kg (42.85 liters) of fuel while the second took 35.875 kg (42.18 liters). So the fuel consumption in the first case increased to 30.18 l/100 km, and in the second case it almost did not change and amounted 29.7 l/100 km.

As a percentage, it looks like + 1.5% for measurement errors and + 8.5% for a decrease in the tire pressure.
As a percentage, it looks like + 1.5% for measurement errors and + 8.5% for a decrease in the tire pressure.

At first, 7% does not seem like a big figure.
But imagine an average transport company with 100 vehicles, covering 100,000 km a year (it is not a big distance for commercial trucks). Taking into account the fact that trucks are heavy loaded and roads are often much worse than the Ring Roads, the fuel consumption will be higher than it was in the experiment, for example, 35 l/100 km. Thus, each truck consumes about 35,000 liters of fuel per year . Multiply it by 100 trucks and increasing prices for fuel. And it does not include losses due to decreases of tire service life and transportation safety. Do you really think you are willing to pay such a price?
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