We had the 210 hp version of the 7.8l, in order to obtain 90km/h we ran the engine at 2200-2300 rpm. As noted above these trucks tended to have low gearing with a top speed of around 55mph. Other than somewhat higher fuel consumption (well out of the torque curve no doubt!) it ran smoothly at this rpm. We had no issues on a 6000 km round trip to the Yukon.
Currently we are running a 6.6l 170hp version (shorter stroke), it runs at 2600-2700 rpm to get 90km/h. We considered Re- gearing, however, the low gearing is really useful for us off-road. This truck has an Allison 545 and 2 speed rear diff (5.86/8.17). These “brazilian” engines have been very reliable for our use. I did put new Bosch injectors in the 6.6. It was a street sweeper and probably the main engine was never loaded properly…..