I recently had the transmission replaced in my 2009 Dodge Grand Caravan. This is my third Dodge minivan, and all of them averaged around 17 mpg in mostly city driving. However, when I got this van back with the new transmission, my fuel economy tanked (so to speak). The gauge started at 17.4 mpg, but began falling steadily. Two weeks later, it is showing as 11.2 mpg, but I just refilled the tank and calculated the last tankfull as 10.0 mpg based on the trip odometer.
A few days ago, I got a P0456 code (EVAP System Small Leak). No problem with the gas cap, and the code disappeared a couple days later. From searching the web, it seems like a small EVAP leak can affect gas mileage, but not in a major way.
Since this problem started right after the transmission was replaced, is it possible that there is some causal correlation? Or something else I should be looking into (besides the P0456)?