Originally Posted by Litos
if you're driving ther car normal, you shouldn't see a decrease in MPG.
i was getting 26mpg stock, then went to JB4 and still got 26mpg.
now i'm Cobb Stage 1 and will admit i get @ 23-24mpg ONLY BECAUSE the boost comes on a bit quicker and i like feeling it.
when the feeling of the "new tune" goes away, i'll be back at 26mpg.
on the highway nothing changed from stock, to JB4 to Cobb.
The dynos I've seen in other threads shows the air-fuel ratio (AFR) dropping quickly down from about 15 or 16 to 12 at RPM over 2900 when the boost seems to be maximized, whereas stock keeps AFR around 15 or 16. I think the AFR of just over 12 is the major reason for the much lower mileage with the Cobb when under high boost - about 25% more fuel in the mixture than air entering the cylinders compared to stock. So above 2900 RPM it would seem that fuel economy could easily drop by 25%, say from 28 MPG down to 21 MPG. I think in 6th gear 2900 RPM corresponds to about 90 MPH, so highway mileage should be okay under cruising conditions....
But we all know this. Best fuel economy is achieved with the lowest RPM in the highest gear. Most if not all tuners are looking for maximum torque and power. Wouldn't it be useful to see a tune that actually improves mileage, i.e., keep the AFR higher than stock? Not sure how to extract more chemical energy out of less gas, however. This is probably what the BMW engineers are constantly working on, IMHO.