injectors and pump change MPG numbers?

[breadstick]

A friend of mine has a 2013 STI and put a bigger fuel pump and injectors in his car. We both live in military housing, which means almost all our driving is at 15-30 MPH with very frequent stops. He recently got a built engine and really babied it and didn't go over 4000 RPMs during the break in. His MPG gauge says his average is 34, which seems really high on base, even with driving extremely easily. The highest average I've gotten on base is around 24.

 

My question is will his having bigger injectors and fuel pump throw off his MPG numbers? If it measures pedal response, it seems that it would because he doesn't need to use as much pedal to get the same response. If it actually meters gas consumption, it should measure much lower, right?

[ras06LGT]

The MPG meter is calibrated for the stock injector size and injector pulse width. During normal driving larger injectors can deliver the correct amount of fuel with a shorter pulse width compared to the stock injector size. The ECU sees this shorter pulse width and thinks it's using less fuel, which is why the MPGs read high.

 

So, yes, the larger injectors throw off the MPG meter calibration. It should be off by the ratio of the larger injectors to the stock ones.

[breadstick]

Thanks for the answer. I thought it was something like this, but I wasn't sure. It was pretty upsetting that we live a block apart and work in the same place and he was getting such greater mileage than me.

[Nrw]

Do the math out when you fill up. You'll see what you are really getting mileage wise.