How hard can a 6.2 diesel be run?

[rs74]

Took the diesel for a drive today. This is my first diesel so everything is a little different. This truck has 4.56 gears, 400 trans and is 4wd. I ran down the road and got it up to about 55. It sounded like it was revving pretty high. I didn't push it any harder until I learn a bit more about how these engine run and drive. I don't have a tach in it so I don't know the rpm's I was pushing. With the 4.56 gears how fast should I be able to go without starting to be hard on the engine. It took a bit to get up to speed with the 6.2. I am not used to such a crawl to get up to speed.