05 cummins 04 build date advise

[JimT]

sounds like my truck, top gear at 68 mph is roughly 1400-1450 rpm. 3.36 or 3.42 rears, i keep mixing them up.

I almost never go higher then 12th (direct) on mine. 1550 rpm = 62 mph, 1600 = 64, 1650 = 66 and 1700 = 68. Drop down another gear and 1700 = 58 mph, but the fuel mileage isn't quite as good in that gear. Only time I drop below 1500 is when I'm decelerating or going through the lower gears.

Of course this is an allison auto so you really can't control which gear you're in. So with those rear ends you'll constantly be torturing the engine, lugging it with low rpm torque. Yeah it's how they are being spec'd and built these days and even cummins instructs people to drive this way, but that doesn't mean it's good for the life of the engine. Save a few bucks on fuel but spend a lot more rebuilding or buying new engines.