Every few months or so I keep track of my MPG for a few weeks, just for fun. With my mods, my daily commute (about 475 miles per week) I've consistently never gotten higher than 28 MPG and usually it's between 27 & 28. I have a fairly heavy foot and spend a lot of time above 3000rpms, but I'm not a maniac. I pick my spots carefully, as much to avoid accidents as tickets.

Anyway, one of my mods is an MP3 ECU. The suggested Octane is 91 but since it's not available here I've been using 89. The last two months I thought I'd try 93 and see if it made a difference. Well, it sure did.
I've been averaging 31.57 MPG since I switched. In fact, the lowest MPG I've gotten on a fillup was 29.80.

Over the last 8 fillups it's only cost me about $10.25 additional in gas but I've gotten an additional 358 miles. That is well worth the extra cost.

BTW, I am not recommending higher octane gas for a stock motor/ECU. But if you've got the MP3 ECU it's worth giving it a shot.

 

Very interesting. I also commute daily (about 40 miles each way, 80 round trip), so I may have to try the higher octane for a tank or so and track fuel consumption. We have 3 grades here, 87, 89, and 91. I almost always use mid grade (89), since I get a 10 cent/gallon discount it costs the same as low grade (87). I honestly have never really noticed difference in performance or mpg, but then again I have never really paid that much attention I guess. I'm curious to see if there is any mpg increase w/the stock ECU from one grade to another.

 

I'd say just stick with the 87, since that is the grade of gasoline the car was engineered to use. Also, I'm guessing the 89 has 10% Ethanol blend like we do here in Iowa? Sure it's cheaper, and supposedly higher grade, but I would never put that junk in my tank! I'd rather stick with lower grade non-ethanol gasoline, even if it does save me a couple bucks on gas at every fillup. That's just my 2¢ ...

 

Yeah, what KrayzieF said. The stock ECU will not benefit from the higher octane. I wouldn't even be buying the 93 if the 91 was more readily available here.

 

my p5 with the haltech is getting only 20mpg or so right now but the Beetle TDi is upwards of 46mpg inthe city!