new truck advice

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I'm looking at buying a new truck new to me. I see a bunch of Paccar MX 13's I was reading a little bit about them it looks like the 13th might get a little bit better fuel economy. the other big engine I see is the Cummins ISX 15 I'm not too sure which one is more reliable which one i should buy the other factor on this is I drive in the mountains 90% is in the mountains I'm thinking the Commons isx 15 bigger mass better Jake brake that's just a guess!! wanted to put this out see what you guys thought which one might be better for my situation dry flatbed so I am always heavy and 25% of the time need over Wait permits maybe asking I’m asking the wrong questions