I recently did a manual calculation of mileage instead of trusting the computer, and it seems the computer is high by about 2.5 MPG. The hand calc came out to be 20.5 on my 4-mo with 18's in mixed driving. Based on my searches that sounds about right...

I've got a spreadsheet that I've been running since my car was new and I find that the MFA MPG readout is accurate to almost exactly 1% of calculating by hand (sometimes high, sometimes low - but overall, amazingly accurate). This is over a six month period. Certain fill ups have been further off (by as much as 3%). Your discrepancy is about 12%.

But I'm entirely stock (engine, chip, tip, and tires). Do you think the 18's are throwing it off? Any other mods for you?

 

I bet the 18' are throwing it off especially if the wheel tire combination is close to the stock diameter for the 205/55/16 which is 24.9" . The ECU will think it is as stock

 

I run 225/40/18 which is about 25.1", or a difference of <1%, so circumference is about 3% off. I guess there's still about 10% unaccounted for.
The only other engine mods so far are an ITG foam air filter and removal of the screens. I wonder if it's seeing too much air and running rich.
Could one correlate discrepancies in mileage with incorrect A/F ratios (on a bone stock car, let's say)?

 

I agree with Altair 4. I'm in sales so I keep records of everytime I put gas in the car, and I reset the trip computer each fill up (I always fill up). Then I check the computer's average mileage readout, and it's almost always within a couple of thenths of a gallon. Doesn't matter what type of driving I'm doing, the thing is deadly accurate. My car is mostly stock, I do have a neuspeed chip. Oh, I removed the center rear headrest also, I'm sure this mod helps the computer's accuracy. :lol:

 

I agree with Altair 4. I'm in sales so I keep records of everytime I put gas in the car, and I reset the trip computer each fill up (I always fill up). Then I check the computer's average mileage readout, and it's almost always within a couple of thenths of a gallon. Doesn't matter what type of driving I'm doing, the thing is deadly accurate. My car is mostly stock, I do have a neuspeed chip. Oh, I removed the center rear headrest also, I'm sure this mod helps the computer's accuracy. :lol:

I retained the center headrest, so I think that discounts your comment on computer accuracy. But I'm positive by removing it you have improved throttle response! :lol: :lol:

 

There is an adaptation for the MFA fuel mileage. You can adjust it in a range of 85 to 115 percent with 100 percent being set from the factory. If there is someone local to you with a VAGCOM they can adjust it for you.