Looking at the gearing, tire selection, and comments from Cummins on what's the ideal operating range, it seems that out trucks are primarily designed to haul heavy loads at 65 mph. Even the tires are maxed out at 65mph with a maximum load. For running light loads at higher sustained speeds a change in overall gearing seems in order.

 

These trucks will live for ever...well past your expected lifetime. Speed on! It only costs more in fuel.

 

IMHO 80 mph isn't really fast and definately not hard on these Cummins unless you're running like 21" tires and 4:56 gears
Low speed, stop and go, trips less than 5 miles, that's all what kills a motor.

I'm almost always driving by the tach, keeping it around 1900-2100 rpm, with my drive line thats about 70-75 mph. Mainly to preserve the 17-18 mpg range

 

I have 315/70R17s with 48RE Auto and 3.73 gears.

My RPMs are a little lower at 1975-2000 I am at 78-80 MPH, which is normally where I set the Cruise Control when empty. In Texas where I drive this means you better be in the right(slow lane) most of the time.

I had the computer RPM reprogrammed for the larger tires. I had the dealer set it to 605 RPMs. I have checked this multiple times to see if the odometer is correct at this setting and it is.

Dont know why the difference.

 

I purchased mine to cut my fuel costs. I try not to over 75mph. Any thing over 70 kills my mpg. Plus, I've killed more than my share of Deer, with many different trucks over the years... So speaking from experience I can tell you that stopping these trucks short from 55mph works alot better than 80mph.

 

75-85 110 miles a day

what fun