Looking at the ESM it appears the MPG calculation is partly determined by electronic pulses from the engine. I read somewhere that this Hitachi sensor is used for both position and speed. I wonder if the recalled sensor is the one used in the MPG calculation and if the new sensor will correct the 3~4 MPG error.

 

I doubt it. If the engine sensor(s) weren't functioning properly, your car would run horribly if at all since they need to be very exact at all times. In the case of the TSB, it is a preemptive measure to replace the sensors before they fail casuing other problems when they do so. The MPG calulation is something I believe they could easily fix with an updated ECU since the computer knows all of the information it needs to do so. Why it isn't doing so correctly is beyond me. That being said, replacement of the sensors will likely have no effect unless they were broken already and like I said before, you would probably know if that was the case.