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again that is because the mack engine is by far and away more powerful then the cummins. The auto transmission behind the mack would be an even better combo but you will not improve performance behind the cummins unless you can stay in the sweet spot all the time and that is what the more gears...

Yep I agree, the mack engine will flat out run away from an NHC250 plus being a tractor it also has the weight advantage. You will be sorely disappointed if you put a 5 speed behind the cummins, you will not gain much if anything at all.

That truck does not have a 400 cummins it has the 250 cummins or the 8.3 cummins I don't remember and yes its an over the road transmission. There are a ton of models rto rtlo rtoo, the last two letters are more of the design, the O stands for overdrive, the L stands for low gearing which means...

I agree completely, get an eaton fuller 9 or 10 speed and enjoy, you will like it much better than the 5 speed.