It works well for the military use case, because they are almost never running over 45mph. Its got enough umph to pull small trailers and can still do basic highway stuff, but its primarily used for off highway and trail type use. It's not a great engine and the HMMWV isn't a great vehicle, but they do OK. The newer Up-Armored HMMWV's that got the Turbo 6.5/4L80E combo do MUCH better and didn't seem to have any less longevity/durability. You could easily rock them at 65mph on the highway and that was in a 15,000lb vehicle. The real diesel engines combined with the Allison 6 speeds in the mine resistant vehicles put the HMMWV to shame in every respect though. I worked as a Field service rep on the MATV for a few months in Afghanistan and they are an absolute beast. Of course an engine/trans combo in one of those things was about $50k, but it would pull down 20mpg and pull a 21,000lb vehicle at 65mph like it was nothing. 18" of 4 wheel independent suspension travel, automatic locking diffs and independent tire pressure control to each tire.
You could accelerate from a dead stop through 45mph up a 60% slope. They had new crews go up a 60% slope in driver training so that they could get a feel for it, since most were transitioning from the HMMWV that didn't have enough power to even go up a 45% slope. It would start going up something like that and just bog down unless you popped it into 4Lo. Heres a video of what that 60% looks like, its a student driver which is why its sounds like it's struggling but really the driver is just spooked and barely giving it any throttle since it feels like you are just pointed up at the sky.
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