There are many differences between domestic US trucks and international trucks. One of them is that international truck mpg (miles per gallon) figures are rated much higher than their US counterparts for the same class and type of truck. This specific difference is due to many reasons.
These reasons are caused by many differences. One of them is that domestic trucks plying in the US are made by US-based automobile manufacturers. These manufacturers follow SAE (society of automotive engineers) standards, whereas trucks made in Europe follow European standards and trucks made in China and Japan follow their own standards.
Differences in different standards cause a change in the fuel economy of vehicles manufactured in the US and those manufactured elsewhere. Part of the difference is due to different criteria specified in these different standards. However, the common aspect about the different non-US standards is that most of the criteria result in manufacture of similar class of trucks having almost similar mpg characteristics.
US manufactured trucks generally lag behind slightly in actual mpg of comparable models of trucks. Part of this is also due to the influence of the big three of the US automobile industry namely GM, Ford, and Chrysler on the US federal government.
These companies have invested far too much in their sheet metal based plants to make drastic changes in the process and materials of manufacturing. This is why their truck models are heavier. Therefore, they have a lower fuel economy than most European and Japanese truck models. Japanese models especially have incorporated more material-related and other technological innovations to reduce truck weight and fuel economy.
Part of the reason US trucks are heavier lies in the fact that they are equipped with more powerful engines. More powerful engines guzzle more fuel and result in lower fuel economy. Even these US manufacturers cannot be blamed because US consumers of trucks have had a traditional and specific penchant for powerful vehicles. However, with gasoline prices climbing beyond the $4 per gallon mark, this is set for a change as fuel economy is having a greater impact in the US market than it had earlier.
The US EPA (Environmental Protection Agency) has also tried to correct the course to actualize truck mpg ratings by dumbing down its earlier published ratings to bring them in sync with practically achievable figures.
It rates the mpg of various current trucks and cars plying in the US on an annual basis. It classifies different vehicles under different classes. Even custom configurations of hybrids and other trucks are rated by the EPA.
The traditional outlook of people in the US towards truck fuel economy is also one reason for the difference. Moreover, people in the US have traditionally had far more money to worry about high truck speed and its consequent adverse impact on fuel economy. International truck mpg figures are more realistic and better. This is because of the awareness of the impact of crude oil price rise and operation of trucks at lower optimum speeds to achieve higher fuel economy.