I thought I'd share for the first time the latest results on my miles per gallon data collected on my 1997 Jetta Trek (2.0L).
Quick summary: I have meticuously tracked MPG since my car was new to its current 92,000 miles. To keep consistent, at each fillup, I ALWAYS fill the tank to full, and once the pump clicks off I stop - never topping off. While pumps will slightly vary when they stop, this is the only way to maintain consistency without always adding a specific gallon amount. Gaoline usage is then entered into an Excel spreadsheet.
OK, the data
Over the roughly 92k miles, I have an overall average of 30.20 MPG.
Mobil: Avg. 30.50 MPG (93 fillups)
Sunoco: Avg. 29.98 MPG (96 fillups)
Shell: Avg. 30.05 MPG (9 fillups)
Exxon: Avg. 30.33 MPG (42 fillups)
Others: Avg. 29.24 MPG (10 fillups)
Analysis - while it looks like Mobil is superior to all other in terms of MPG, the data is not what it seems. Sunoco was near my job when I got the car, and at that time it only averaged 28-29 MPG while it was breaking in. Mobil is where I go now most often and now I rarely get below 31 MPG per tank, even during current Sunoco fillups. The others category are generic stations and Gulf when I was desperate, which I do believe have inferior gas as the data shows.
Super (93 octane): 30.67 MPG (29 fillups)
Plus (89 octane): 30.09 MPG (124 fillups)
Reg (87 octane): 30.21 MPG (108 fillups)
Avg. MPG just during heavy A/C use: 31.59 MPG
The data used to show a definitive curve of about 0.5 MPG at each grade level, however, regular has surpassed plus. Although once again the data may be skewed as the car is breaking in. With gas prices being high. I went back to just regular last spring and have experienced great mileage in the 31-34 range each tank full since then. However, I would like to switch to all super for a while again to see the difference now. When my car hit 85k, I truely felt like it was finely broken in. The engine just seemed smoother, easier to rev, and actually it felt more powerful.
Some other notes, gas mileage goes down about 1-3 MPG in January and February during real cold periods (real cold periods here means never getting above 20 degrees). With the data graphed, there is a very visable trend of increased average MPG as the car gets older. The highest MPG I have recorded for a tankfull is 39.52 - with 2 bikes on the roof rack. This was attained during a complete highway trip on 93 octane gas, and I believe a strong tailwind.
Finally, if you're interested in my driving specs, I drive a mixture of highway/city with a split of probably 60/40. I will see a very slight increase if I take it easy over a full tank, but even if I'm driving hard, foot to the floor acceleration at highway on-ramps, etc...I don't see any noticeable difference. As shown, air condition use does NOTHING to reduce my mileage, and in fact I get some of my best results with the A/C on!
Thats it - its a little long but I thought some of you would find it interesting.