I have lately been on a crusade to improve my gas mileage by doing all the usual things, (new plugs wires, pcv valve, o2 sensor, etc.), as well as opening up the input pipe to the stock airbox. Being in California, we have this crappy gas were stuck with that is oxygenated using either methanol or MTBE. This is known to reduce gas mileage and power, so I have to keep that in mind. I found that 76 uses a ethanol mix, but I believe, not sure Mobil and Arco use MTBE. In any case, I'm trying to determine which gas brand gives me the best gas mileage, since this is mainly a commute car for me.

I just finished a trip from San Diego up to San Jose, and a side trip from San Jose to San Fransisco and carefully recorded gas mileage, and I got some interesting results. Heres a sequence of events:

Before leaving San Diego, I filled up on 76 gas. Going through LA, I got stuck in an hour long traffic jam, but all highway otherwise. At arrival to San Jose (468 miles), with one 4 gallon fillup in between, my total gas mileage was 25.7 MPG. I filled up on Arco in San Jose. Average ambient temp between 85 and 95 degrees.

The next day a made a round trip to San Fransisco (about 168 miles). After that trip filled up again with Arco. Gas mileage was 19.5MPG, this was no traffic jams, and 95% highway driving, no steep hills. Average ambient temp was 75 to 85 degrees.

On driving home to San Diego, I added 4 Gallons in LA, and then filled up in San Diego at arrival (473 miles). There was one traffic jam of around 30 minutes in LA, the rest all highway. Gas mileage for total trip 28.5MPG. Average ambient temp was 90 to 100 degrees.

So now I'm thinking, first why such lousy gas mileage in San Fransisco trip? Was the computer relearning from 76 gas to Arco, or was it the lower ambient temps, or both? The trip back was also much better than going up. Was it higher temps helping or is Arco better than 76 (MTBE vs ethanol blend)? Needless to say, I'm still a bit confused which gas to use, so will continue to experiment. I'm also thinking, maybe cooling the air intake is great for power, but not so good for gas mileage. I'm going to try taking off the pipe in the front of my airbox, and let it suck engine air directly and run a tank through that to see if it helps mileage. I'm not as interested in power right now as mileage. Anyone else have some insight to share on this?

In the Northeast MTBE was banned and I noticed with the new ethanol mix
my gas mileage has improved.
Usually to get accurate gas mileage readings you should go from full to full at the same station same pump to figure out the gallons used. There is allways differences
in pump calibration from station to station and how much you top off the tank that
effects the readings.
Yes different grades and brands of gas could cause the PCM to re-learn based on how the fuel is burning.

I did research some more and I do believe MTBE was also banned in Cali as well. My initial comments were based on the fact that the pump at 76 stated it used ethanol, but the Arco/Mobil don't show that on their pumps in an obvious place. Another thought is maybe the gas from San Jose (Nothern Cal) maybe uses less ethanol than San Diego (Southern Cal), because Southern Cal has a much worse smog problem in general. The difference between the two trips amounts to 1.6 gallons which I think is larger than what an error in fillup would cause. Another possible cause is wind direction. Maybe the prevailing winds are blowing from the North. There are so many possible variables.

You got it. There are so many variables you can't expect the exact same results every time.