I have had Air Lift air bags on all our GM trucks since 1993 both for my business and personal use and they are superb and very predictable. NEVER leaked air even once on any of them with having a combined trucks with more than 900,000 miles of combined loaded travel on the bags. 100% adjustable for any load rating up to 5000 lbs.
For the business the trucks carried what ever was needed and for personal use my trucks were only used since new for towing a heavy Carriage 5th wheel (12,800 lb truck scale weighed loaded to travel) with 2460+ lbs pin weight plus having the 106 gal aux diesel fuel tank with a hand operated fuel pump full in the bed too (another ~750+ lbs on the rear axle) or our hauling our loaded 4000+ lb Lance 11'4" truck camper on the back and again, the trucks were only used for RV'ing and were all bought new.
Simple to calculate the proper PSI needed to carry what load carried.
2 rear airbags on my truck (1 right and 1 left) have 5000 lbs total capacity at 100 PSI inflation which means approx. 500 lbs of lift support for each 10 PSI of inflation in the air bags. Thus, 4000 lbs carried in the bed means ~80 PSI inflation in the bags to have the total weight supported by the air bags alone and 3000 lbs carried means ~60 PSI inflation in the bags.
You really don't want 100% of the carried weight to only be totally supported by the air bags alone so to have roughly ~1000 lbs carried by the truck springs which usually levels the truck's rear you should inflate the bags to 60 PSI for 4000 and or 40 PSI for 3000 respectively for an excellent ride quality and around a true level truck setup. Very little to no sag from being the truck's near unloaded as about all pickups when totally empty (OEM) ride higher in the rear than in the front. Don't want the truck's rear higher than the front!
It works excellent and has since 1993 with no guessing needed. Know your actual additional load being carried within 500-1000 lbs and inflate accordingly.
Each 500 lbs on the rear axle should require an additional 10 PSI inflation in the air bags minus what weight you want to be supported by the truck's rear springs! How easy is that?
Solid rubber type springs have no easy adjustability other than grabbing wrenches and climbing under the truck to reset contact position for various loads as we used to have them prior to air bags and never will again. Truck loading always varies from driver alone to many passengers plus a heavy cargo load on the rear axle. Air bags solve this issue so easily and quickly (maybe 30 seconds with an air compressor or air line or in about 2 seconds if you have the onboard compressor option installed which lets you control bag PSI on the road even while driving.
A superb CC LB 4X4, GM HD Diesel, airbags, Rancho's, lots more
Lance Legend TC 11' 4", loaded including 3400 PP generator and my deluxe 2' X 7' rear porch
29 ft Carriage Carri-lite 5'er - a specially built gem
A like new '07 Sunline Solaris 26' TT