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)?

  • Have you checked the transmission ratios?
    – Solar Mike
    Jan 27 '20 at 18:06
  • 1
    Obviously the answer is yes, the transmission could impact the fuel economy. It's possible that there is some internal problem in the transmission that is causing excess drag and which will eventually cause a failure. That wasted energy has to go somewhere and it will end up as heat in the transmission fluid. Have you noticed that the engine is rev-ing higher than it used to in normal driving? If so, there may be a shifting problem like failing to go into OD or, as noted already, the final gear ratio is different from your OEM transmission.
    – jwh20
    Jan 27 '20 at 21:06
  • Maybe the overdrive in the transmission is not working?
    – Moab
    Jan 28 '20 at 16:42
  • I called the transmission shop and they said it's possible the transmission is not shifting into high gears, and they want me to bring it in for a road test. Because I have been doing only city driving, I haven't taken the car above about 35 mph, so I wouldn't necessarily have noticed that it's not able to shift into fifth or sixth gear. On the other hand, if I haven't had occasion to use the high gears, would that have impacted fuel economy, since the engine wouldn't have been revving excessively?
    – Rege1
    Jan 28 '20 at 20:10

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