1999 N14 525hp rpm calculation check

I'm trying to estimate rpm and mpg of this setup for otr use.

2000 kw w900. 1999 n14 525HP. 18 speed eaton fuller. 2.85 rears. Tire size standard 22.5.

Cummins website said you want rpm to be either 1450 or 1550 at 65 mph. I thought this was a pretty low gear ratio but I did the calculation and it came out to just over 2000rpm. I did rpm = 65mph X 2.85 X .73 X 983 tire Rev/mile. Then the number seemed high so I divided by 60 minuts to go from rotations per hour to rpm. Seems like I did the math right but this is my first time doing this calculation. I got the equation from the cummins webpage for the n14 (pulls up on Google easily) that said the reccomended rpm.

Could this be true than an 18 speed and 2.85 rears is still simply not a low enough rpm for maximum efficiency with this engine? I would have thought a 2.85 n14 would be a good setup and compare to a detroit 12.7 with 3.36 3.55 etc. I'm between these two motors. Any insight on why my rpm calculation is so high and how you could actually get to manufacturer recommended cruising rpm for 65mph?

 

No I'm looking to purchase a used truck with those specs and wondering why the rpms at cruising do not seem to be in line with cummins recommendations about 500 rpm too high with 2.85 rears. I thought this was fairly low axle ratio so I'm confused.

 

Those are the specs of one truck I'm looking at. I'd prefer a 12.7 tbh but the n14 seems like a good engine if I could get a good rear axle ratio (lower) with a 18 speed setup

 

I hear the 12.7s like to run higher rpms is this why the N14 demands a lower ratio

 

Ok thanks for the help I figured I made a mistake somewhere. Do this would be a decent setup for a n14 then? I use to haul water tankers with an 18 speed in NH which is sometime very hilly and always had plenty of gears to choose from (granted it was a 2004 mack and a newer WS setup with presumably a higher gear ratio). I suppose pulling in 16th or 17th wouldn't be a problem and allow me to get better mpg if I'm empty having the option to lower my rpms when power not needed.