Yes - combined capacity.
Here’s the truck one of my son’s drives - the Trucker’s Truck, the “329 Peterbilt” = “Fuel capacity: Twin 150-gal. tanks
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http://www.etrucker.com/apps/news/article.asp?id=56758
Most places, if you don't meet your delivery slot - they can make you cool your heals from hours to days - and if you're running reefers, you have to keep them going... among other pitfalls
Nice turk. 300 gallons @ 4 bucks a gallon, $1200. Whew!