So i'm curious here... My car has averaged 21/27 since I got it (20/25 when I'm driving like a punk). I threw in some 390s last week, and as expected, the car dropped ~1mpg around town, threw a rich code (which I reset) and held that average for the week.

This weekend, Jan worked his magic on the car. On the drive home I got 4+MPG better than I've ever done on the freeway. Around town i seem to be avg'ing 2 better so far. I'm trying to determine if the mileage calculator is still valid after upsizing injectors.

What i'm confused by is... Obviously the OBC isn't going to know what size injectors I have, but does it base mileage on pulsewidth or is it more of a long-term trim from the distance vs. fuel sender change?

If it's pulsewidth of the injector, and it's running shorter PW since the tune, I could see a lookup table in the ECU having preset mileage values depending where in a pulsewidth map the car was running.

If it's fuel level vs distance, injector size becomes irrelavent... and that means I can trust my mileage computer. If it's PW, I need to figure out a correction percentage to keep my numbers in check so I don't have to go back to the dark ages of trip odo divided by gallons at each fuel stop

I tried googling, didn't end up with much of value. Anyone have any thoughts for me?

 

i think some one said around here that its based off fuel level and miles.

 

When you install injectors with a different flow rate, it will throw off the MPG calculation of the OBC.
The OBC determines the MPG by looking at injector pulse width, RPM and vehicle speed. With larger injectors, the pulse width will decrease for a given engine load. Therefore, you can expect to see a corresponding increase in the displayed MPG. Your actual MPG should not change much at all.