Sweet spot

Need more info, also makes a difference if it's a 12.7 or 14. Probably a 14 with overdrive from that year. It doesn't have a DPF restricting the exhaust, so running it a little higher RPM at less boost won't affect it as much.

I'm not sure what rpm is considered sweet anymore to a Series 60, used to be 1400 rpm for economy and 1600 rpm for performance. It's probably not that much different unless someone has put too many restrictive parameters into the ECM.

The 14 liter I'm driving now has much more low end, it pulls better at low rpm than almost anything else I have driven. I have a lower final ratio overdrive with a 3.42 ratio and 22.5 low profile tires.That's what I'm driving now, and either let it lug down or shift to keep it higher in the power band if it's hilly and that load needs to get there as soon as possible. It pulls hills much faster longer at 1600 or higher at the bottom of the hill than less than 1400.

Lugged down, you are doing low 50s at around 1100 rpm or so before shifting, and if you can see the top of the hill close, you don't shift even then. 3.55 would be better, and 3.70 wouldn't be that bad as long as you drove it 4 to 5% slower. My truck stays below 1400rpm with the cruise on in high, loaded heavy or otherwise pulling hard. 66 mph to maybe 67 actual, not what the speedometer reads. An upgrade of any length slows it faster than other trucks I have driven.

A direct would change things considerably, also tall rubber versus low profile would change it some.

I have driven a Cummins ISX with a 10 speed direct and 3.55 rears on 22.5 lp tires, the sweet spot on that was less than 60 mph, but it pulled well and didn't need to be shifted much on rolling terrain even with hard pulling loads.

 

My 14 keeps on pulling well down into the high 40s at under a 1000 rpm, but it is counter intuitive for me to run it that way. It will also pull past the 1850 rpm peak power.

Any engine should have the overhead adjusted like the manufacturer suggests and that will help everything more than having a 9% higher axle ratio hurts it.

To the original poster, if you run at 1400 rpm or thereabouts in high gear, and are happy with the speed attained, it will probably do well for you if the engine is running right. Reflashing the ECM to new manufacturer's specs has helped, not hurt, the one I drive.