So I had the small bottles, PM22A, then got smart and bought the gallon jug PM22GAL.
Both say they are for ULSD vehicles, both bought at Ford. Almost identical wording except....

PM22A says 4 oz per 25 gallons
PM22GAL says 4 oz per 31 gallons.

Why the difference?
This question has been ask before but has always been sidetracked and never answered.
Ford says the only difference in the part number if for the container size. Google PM22, you will see so it's not a different product.

 

The person who authorized the label wording. Probably difference people and/or different companies/locations bottling the additive.

Once the dosage is exceeded, there is no additional benefits to adding more. Just use the 4 oz and not worry about the difference.

 

I've purchased both the gallon and the 20oz bottles from Amazon. The current case price for the 20oz bottles is cheaper per ounce than the gallon container so I don't bother with the gallon jug any longer. I've always used 4oz per tank (26gal tank) because that is what the markings on the side of the bottle are labeled at. The most I've ever been able to cram in the tank was 22 gal. I've even measured out 1oz to treat a 5 gallon can a time or two. I don't worry to much about how accurate it is, as long as there is some PM-22 in the fuel it helps me sleep better at night.

 

Gallon at Ford was $48.
20 oz bottles are $10.

Eight times as much for five times the price.

48 gallon tank here, love it!

 

That's a good price on the gallon jug. Amazon is asking $68, think I paid $60 a year ago. dieselfiltersonline has the best price but to get free shipping you have to spend $250