A few points to consider. A few years ago, there was a thread where everyone posted their trucks average miles per hour for the life of the truck. I believe there was a feature that was on the older trucks where you could get total hours run and then you had your odometer. IIRC, everyone was really close to 40-43 mph average. The overwhelming opinion at that time was that this was the reason for the similar mpg. The trucks just didn't average fast enough over their lifetime to make a difference in fuel economy between the gear sets.
As far as the "increased wear and tear" due to the higher rpm's turned, IMHO, this is not even an issue. How many internal engine failures do you see posted here? Most people trade their trucks in before they even really get the engine broken in. So look at the hot shot haulers. I am making an assumption and please correct me if I am wrong, but I would believe most have 4. 10 gears and are doing way more miles in a year than many put on the entire time they own their trucks. EB and Tulsa Okie have almost 400,000 miles each on their trucks and have never had to go into the motor for anything. Not to say that at 1,000,000 miles you might see a difference in truck longevity between a 3. 55 and 4. 10.
Another reason to highly consider a 4. 10 is that LenB is looking at a SRW vehicle. At some time in the future you might consider going to a heavier 5th wheel and you may want to run 19. 5 tires for increased weight capacity. This is my current situation. With my 19. 5 tires, my 4. 10 rear end is effectively now a 3. 89. I am glad that I didn't start out with the 3. 73 because I believe I'd have a harder time towing my 5er with an even taller effective ratio.
The last point I've got to consider is EGT's. While towing, keeping the rpm's up keeps the turbo lit and moving more air and keeping the EGT's down. You will read of engine failures, although still rare thank goodness, of people burning up a cylinder because of high EGT's. Comparing a stock truck to stock truck towing the same trailer up the same hill in 90 degree weather, the 4. 10 will run a bit cooler than the other gear sets.
LenB, any of the gears will pull your current set up without a problem. 8,000 pounds behind the Cummins is like a flea on a dog. It'll never know it is there. Just ask yourself if you see yourself in the same trailer in ten years. If the answer is yes, then get whatever makes you happy. If there is even a question about it, consider strongly the 4. 10. You wouldn't wan't to limit yourself 5 years from now for a decision you make today. You are buying this truck for the purpose of towing. Make it the most capable tow truck you can.
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