Best 2 fill ups in the 09 suburban are 19.3 and 19.1. Completely stock and running 30-32 psi in the tires. Roughly 49k miles on the clock.
That tire pressure is much too low, will wear the tires, lower mpg, and may heat up the tires too much. Least I'd run is 35, better to be up in the 37-38 range.
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2010 Tahoe LTZ completely stock - at 70-74 mph long highway run got 18.4.
On board computer mpg numbers are rarely reliable. I've found on mine that the computer indicates mileage about .6-1.1 higher than actual mileage. To me you should never trust the computer beyond some basic comparison while driving, the only way to know true mpg is to fill, drive till close to empty, and re-fill. Even better when calculated over 2 tanks figured together.
25 mpg at 70 mph extended highway driving is not possible in a Tahoe/Yukon with 5/3.