I travel up to the Colorado high country with my Ford F150 (usually loaded with tools and materials). Heading west on Interstate 70 coming down from the Eisenhower Tunnel is an 8 mile stretch with a consistent 8% downgrade. Am I better off dropping to second gear (it’s an automatic with D,2,1) and setting cruise control to 60 mph leaving the truck at a consistent 3,800 rpm. Or is consistent break pumping better?
While a constant 3,800 rpm shouldn’t hurt anything, I can’t say I’d be comfortable doing this with my SUV. If you have a button that locks out overdrive, I’d do that and then use the brakes. Locking out overdrive on a 4 speed automatic essentially puts you in 3rd gear which would be better for the downhill run that going all the way down to 2nd.
Just remember, brakes are made to be used and to wear out. Replacing them is a relatively easy and inexpensive job. Transmissions are made to last as long as the vehicle, they cost a lot and repairing them is expensive. Ditto engines.
You decide.
If it is a manual transmission it should hurt nothing and be safer. If automatic, I will hedge my vote. I will wait for Transman to offer his opinion. I am a manual person myself. I have never owned an automatic. I believe it should not be a problem, but I don’t trust my own advice on an automatic under the stated conditions. 
Brakes work by turning energy (car’s momentum) into heat. That is a lot of heat you are talking about. Overheat the brakes and then you have reduced braking, not good on a long downhill stretch.
I used to train mountain school bus drivers here in Colorado many years ago. You should be in the same gear going down hill as you would be going up hill (automatic or manual transmissions). This holds true for buses, trucks or cars.
Twotone
Automatics are just as capable of providing braking as a manual and often with less, not more distress. Maintenance is the key. Whether you use it hard for engine braking or acceleration, it’s considered heavy wear and more frequent service should be expected. That it’s easier to self service a manual is where it shines. So, I would down shift but not use cruise control for those eight miles and allow for lower speeds as the situation arises. I wouldn’t use cruise for required lower gear ratios on level ground either. In either case, moderate use of both brakes and downshifting instead of one or the other as designed by the manufacturer is always preferable.
A key in this equation is not loading the pickup with so many tools and so much material that you tax the systems.
It doesn’t hold true for trucks anymore. This rule has been updated.
With new semis, they recommend dropping one or two gears before descending the hill due to better engine design and better aerodynamics.
Also, most cars can climb a major hill without downshifting, yet if they are heavy enough, they need to drop a gear or two to maintain control going down the hill.
I’d turn off cruise control first and foremost. While most say transmission are expensive and brakes are cheap, on long downhill stretches, faded brakes can be more expensive than transmissions.
For engine braking…and just as informational question for the many more knowledgeable trans guys out there; I was under the impression that auto trans were essentially similar to manuals in gear selection in this way. An auto trans with 4 speeds, was equivalent to a manual with 5 as the torque converter acted as the lower gear with an essential variable fluid media (with lots of slippage) more apparent in the lower gear ranges that normally don’t lock it out. Correct ?
You might cancel the cruise, down shift to the 3,800 rpm range and touch the brake pedal enough to trigger the ECU to near totally shut off fuel and close the throttle plate. You didn’t mention the year model but some later models have a function to aid engine braking.
Thanks for all the replies. FYI it is a 1999 F150 and I wish I could reduce the tool and material loading, however that’s why I have a truck 
I’ll try the overdrive override w/ some braking next time.