I have a 2017 Mitsubishi Mirage that has 94,xxx miles. I bought the car new.

I was previously having issues with the fuel gauge not reading properly. Tracked it down to a faulty sending unit. Fuel pump and sending unit was all one assembly inside the tank.... I couldn't find an aftermarket fuel pump anywhere at all and the dealer wanted $900 (parts only, no labor) for the assembly. So, I found a used assembly on eBay. Supposedly it came from a 2017 Mirage also.. it was in a crash (front end) at 21,000 miles and the person was parting it out. I got the VIN # from the donor car and the dealer confirmed it's the same pump as mine, so I bought it... Got it home, took out the old assembly from my car, compared the two and they looked identical, so I installed the "new" assembly. Fuel gauge started working properly, all was fine.

It's been about 2 weeks since the replacement and my MPG has been cut in half since the replacement. I am (and always have been) hand calculating my milage... On my 140 mile round trip commute to/from work I use to use only no more than 4 gallons of fuel meaning I was getting between 36-42mpg. Since the replacement I'm using almost 9 gallons meaning I'm getting 16-20 mpg... A huge cut.

No check engine light or stored DTC's. Car is running and driving perfectly.. exactly the same as it was since I bought it new. No leaks anywhere. I went back and checked my connections at the "new" fuel pump assembly and everything is sealed properly with no leaks. Nothing dripping on the ground. No smoke at all. All maintenance is up to date; spark plugs (OEM) and Air Filter were done about 3-4,000 miles ago, before the fuel pump assembly replacement.

I'm at a complete loss... Any ideas?