There was an article in the Wash Post, probably syndicated anyway, upshot of article was the balkanization of fuel formulae across the HEW HESS HAY courtesy of those fine folk at EPA and the ever popular ethanol/farm lobby.

Oxygenated fuel is required in some places, not in others is an oversimplification but will do for starters. In MD, for example, the Baltimore area requires a quite different fuel cocktail than Cumberland in the Western end of the state. In some areas of Illinois, another example was that four different fuel formulations were required within a 300 mile distance.

I wouldn't be surprised if the BTU content of the various fuels varied by a significant amount, not to mention the varied operational effects in engines.

No offense to your studies, but there are the many variables in operating a car make it poor for a scientific study of the energy content of different brands of gasoline. My memory is rusty, but I have taken several graduate credit classes on statistics, and the statistcal analysis of your figures would reveal potential errors. As other posters have mentioned, there are many variables to effect your study in a car. Laboratory analysis will control things like quantity of fuel, and temperature during the test. I took a Fuels and Lubricants class while getting my BS in engineering. We sampled the fuel at the local gas stations and ran tests in the school's lab. There were differences in the fuels in octane and specific energy per gallon, but I don't remember anything as significant as the 12% you report. There were significant differences in octane within the gasoline we tested. Keep up the good work!

Oye Pablito!!

If you can't do it easily I have a calculator that will do stats.

e-mail me the numbers if so inclined.

I wonder if you have an "oxygenated" (partly burned already)

fuel from Chevron. Might be worth talking to Chevron's PR folks

and see what they say.

How does gas distribution work? Where I live, small town, there are only two oil companies that supply gas to all the gas stations here.

All of the gas stations get their gas from the same suppliers, the ol cheapo no name and the chevron and mobils.

My guess is that the company gets unrefined gas, refines it and adds the additives for each gas station and supplies it them?

I am saying this because a chevron gas station owner was complaining to the gas company saying that it was selling cheaper gas to texaco and not to him so the gave him the same price (insider info).

I should follow a gas truck when it makes stops at the cheapo gas stations and see if it goes to the goods too.

Hate to nitpick since you're a frequent and credible poster on the BB, so forgive me for doing so...

I wonder about quality control; about the age and supply-turnover among the brands you used. Did you visit the same station each time for each supply? How long was each brand in the tank? Which brand was in the tank first, or did you alternate? Could one brand have affected the next brand? Atmospheric conditions(headwinds, humidity)? Traffic density? Tire pressure? Air Filter?

Maybe you had octane differences(due to quality control) which affected the pre-ignition just enough? Maybe you got less, or more, octane than you paid for???

THis was not a controlled experiment(IMO), and there's no way to do that using just one vehicle -- since you can't use both fuels simultaneously. However, if you could get yourself a precision fuel-flow meter connected to the supply line of your car, you'd get results I'd have more reliance in. Or maybe a dyno test.

However, I understand the Chevron may have Techron in it, which I put in my '93 240 every few months, which quiets the engine(a bit) and gives it a "refined" feeling for a while. I've never noticed a MPG difference by adding Techron. I HAVE noticed an increase in MPG during the first few months of a new air filter in that car.

Anyway, I'm in New Jersey where there's no such thing as Unocal 76 OR Chevron. But Chevron was WORSE for you, so..????

BTW, in my '97 S90 ("My other Volvo is a 240") I just hit 27.5 mpg on the last tank, when I usually get about 24 mpg during my 60-mile commute of mostly freeway driving. My low-fuel light usually comes On at around 450 miles on the trip-meter and that's when I usually take it to about 480 before filling-up; several times in the WInter it came On really close to 400+ which is lousy MPG. This time I hit 500 miles before the light came On and I thought the warning light had quit working until then, but no -- I hit the station at 506 miles with 2.5 gallons or thereabouts still left in the tank. (Capacity of 21.5 gals. and fill-up of about 19 gals.)

I wonder if I might actually hit 28 or even 29 if I follow the speed limits exactly(!) ;->

On the last tank, it was the first time in several months I could remember keeping it in the D position for the entire tank. But I think THAT has something to do with the warmer weather. My mileage usually drops in the Winter and looks like it just came back for the Summer.

Sorry for the long post, but my $0.02.

Seasonal differences (i.e., one supplier during the winter and one during the summer?) How many tanks? How many miles? Ethanol blends? Notice any idle quality differences between brands?

I do notice a BIG difference in idle quality from various brands around here in Iowa. Amoco works fine. Local convenience stores' gas quickly clogs the injectors and idle goes down. I am told that Shell in California has had similar problems.