In commercial vehicles the curves of road resistance are dispersed over a wide range (see figure) due to the large variations in laden and unladen weights and the large influence of the vehicle weight on the total road resistance. Hence a basic layout of the maximum speed corresponding to the above-mentioned cases is not possible. The rear-axle layout is thus chosen in such a way that at the most frequently driven speed, as good a total efficiency as possible can be realized. In long-distance trucks for example, the most frequent speed is the maximum speed of 80 km/h (50 mph) prescribed by law. The figure shows a typical layout for a truck.
The unladen truck can accelerate in the plane up to a speed limit of approx. 120 km/h (75 mph). When completely loaded, a speed of approx. 105 km/h (65 mph) can be achieved at slight underspeeding. The direct gear cannot cope with the gradient represented by the road resistance curve on the left. Using a smaller gear however, the full engine power can be used and the theoretical maximum speed of approx. 85 km/h (53 mph) be reached.
The total transmission layout has to be examined from the point of view of the accelerating and climbing ability in the individual gears. The excess traction forces and powers resulting for the respective gear ratios have to sufficiently meet the driving demands.
The achievable driving powers can be checked by drawing in the demand curves for climbing or acceleration in the traction force or power diagram and by analyzing the excess traction forces and powers available in the gears. In this way, large gaps in power can be avoided, which would require frequent shifting from the driver.