I'm sure there's a documented amount of fuel saved with the start/stop system, but exactly how much for a given duration of driving is a data point we'll never pry from FCA. Just like their artificially running the 8-speeds about 20 degrees hotter than a lot of auto trans guru's feel comfortable about to eke a few hundredths of a mpg more, or the hemi shutters. Hell, on one of the 2019 Ram 1500 description sites, RAM actually has engine coolant plumbed thru the differential cover to warm that assembly allegedly in the quest for the mileage grail. If I didn't see that, I wouldn't believe it.