3406A w/ 15 spd.

not sure on the 3406a but my brother has a 3406b turns like 1750 at 70 been like that for years

 

Never driven an A model but that doesn't sound odd at all for an older mechanical engine's normal operating rpms to be 1700, 1800+ that's just how they are. Try lugging them slower than that under loaded condition, driving it like a modern electronic engine at 1200-1400rpms, and you'll ruin the motor in short order.

 

the old big cam cummins like high rpm. actually even higher like 2400. the cats are tprque motors and even the mechanicals are better in the lower range. i learned on that motor and i never ran it over 1600rpm. our trucks were west coast trucks with cats, 15 or 18 speeds and rears in the 3.30 area. ran just right at 70

 

The old A's like to run between 1200 and 1400. It's differant than the old 855 Cummins. Is it the 290 or 350? Just a warning don't let it overheat they are prone to crack heads. Had a 290 with a 350 turbo and CAC wich I turned up to close by 420 horses, ran with standard Celect Plus N14's all day

 

My 1st truck had an A model Cat and at highway speed 60 to 68mph it turned between 1500 and 1650rpm's. It had a 13spd, tall 24.5's and 4.11 rears. Good motor but hard on fuel. Mine had a preheater on it which came in handy in the winter.

 

seems to me my 80 cabover with a 425 was an A might have been a B too long ago but 1600 was the rpm at 70 mph 330 rears