Ok here is my question, I was custom tuned by Al, car runs great, and has for months. Tuned at Buschur for 93 oct w/ 10% methonol, like most gas station around here in MI. I have been getting 22 or so MPG freeway/street/hard driving. I run a 10.8 AFR at WOT (rich intentionally) at 350whp/302wtq, HKS 272s, ATP GT3076R Turbo and a custom intake, 2.5" LICP, 3" LPI exhaust catless, fuel pump, Forge unos MBC and recirc RS BOV. BPR7ES plugs gapped to i think .26. Now no issues so far. I put in about 1/4 tank of 94 octane that is at my regular gas station in the car for the hell of it, and procedded to beat on the car MUCH more then usual (just did a clutch line/restrictor removal/synthetic valvoline fluid flush) so I wanted to get used to shifting hard with this set up. Due to the price of 94 being 2.95/gal (bloody steal if you ask Dubya) I went to top back offbefore I went in for the night. And as I often do, I averaged my mileage and it was 30 MPG........... i.e. 59 miles driven on exactly 2 gallons..................... So my question is WHY?????? Did the 94 oct lean me out a bit? my idle seemd to have a -18 vac compared the the -14-16 vac before...... any ideas? Thanks

NOT THAT I AM UPSET

 

To really make a fair MPG comparison between the two fuels, external conditions and driving behavior must both be exactly the same for each.

 

Well thats true, I would say yesterday was 78 deg, and 25% humidity and driving style was much harder then usual, mulitple WOT runs and 15+ lbs of boost on any "calm" driving. The last sience Febuary temps have gone from 20's-90's at some point driving mostly calm, little boost/daily driving. So why driving harder now w/ 94 makes such a diffrence?

 

bump

 

higher octane gas should have less mileage.
Best mileage happens at 65 degrees temp.

 

This is a lot of work, but if you could data log long duration runs at cruise may it may tell you more about the two gas grades. Burn speed and octane may cause the computer to change ignition advance and injector pulse as a result while in closed loop at part throttle. Be interesting to see if and to what degree it'd change the actual mileage as a result.

 

i think you cannot base your mpg on such a low amount of fuel filled up.
i mean that when ever you fill up the tank, it will never fill up full the same. for example, a 0.1 gallon difference will bring your calculations down by 2mpg.
fill it full again, and wait till it empties out.

 

ohh and with your higher vac numbers, did u notice a slight increase of idle rpm? at 1k+ rpm idle i usually get 17-18 vac.