Ok guys got my tune a little over a week ago and I'm happy with the power but not with the loss in mileage. Went from 21 MPG to 18 MPG. So my question is this. My tune is right around 13.5 in the low rpm (>2000) stock runs I was closer to 15. Since 75MPH is right around 2000 RPM if I had the tuner lean out the curve under 2000 RPM shouldn't my mileage go back up? I know I'd loose a little power but since I normally leave the line higher than 2000 RPM I don't think it would be a big deal.

Thoughts

 

No, not unless you cruise at WOT at 2000 RPM. I'm assuming that your 13.5 A/F is only at WOT,open loop, so when you are cruising you are normally at part throttle, closed loop. At part throttle, if tuned properly, your O2 sensor's feedback is holding your A/F ratio at 14.6:1, unless you have a problem such as a defective O2 sensor or an improper part throttle tune that would prevent the closed loop function to properly correct to this 14.6:1 A/F. This could easily be checked with a scanner monitoring your O2 sensor and fuel trims at a 2000 RPM cruise.

 

Thanks for the reply. I didn't think about the WOT effect on the dyno. DUH!

So steady cruising should be closed loop around 14.6? I'll have my tuner check it.

 

do you still have your MAF in your intake? or are you Speed density tune?

 

Still running MAF.

 

Your cruise MPG should not be hit by performance
(WOT) tuning. But transient throttle can be, and
bringing PE in early will enrich formerly-stoich areas
like light acceleration for example. Do some cruise
and street driving logs, see if you are stepping into
enrichment more, etc. Take a highway-only drive
and see if there is any real closed-loop, steady-state
MPG hit or if it's all during acceleration (in which case
chalk it up to "entertainment expenses").