Myth Busters
 

I have a myth to debate, here it is:

"Winter gas" has a different, more volitile, formlula that tends to get better milage than "Summer gas"

Some of my friends have told me this as a reason as to why my fuel milage has changed drasticly. Does anyone buy this or have any comments or facts on the matter?

My fuel economy has decreased from 32mpg consistently to 26mpg consistently. I average 400+ miles per week, so the 6mpg is pretty noticeable. The car has not really had any new changes/problems. It has a little bit of a hot start problem that it's always had since I bought it. It also has a new FPR and vacuum lines to the FPR, also has a new fuel filter. Any input is appreciated.

actually, winter fuel will get you LESS economy.

I have a theory as to why you get better mileage in the winter:

You are driving harder in the summer.

nice theory CJ, I've actually tried driving very conservatively for the last two tanks...but it is quite believable.

nynor, I would tend to agree with you...my friends had said winter gas was more volitile...which makes sense...but higher volitility I would think would tend to decrease the fuel economy...still doesn't acount for my milage loss, it contradicts it

I don't know about my 944 but I know with my hyundai I was getting close to 375 miles/tank on summer gas when my roommate told me they were switching to "winter gas". Within a week my miles per tank dropped to about 325 on a good tank. Since then I haven't been able to get above 350 miles/tank so I don't know what to say to that....

The feds require big cities that have high pollution levels to use "oxygenated" gasoline formulas during summer. Less bounce per ounce so you use more fuel per mile, but emit fewer emissions.

not to mention, but you get more gas into the tank when the gas is coooold... it's more dense.

do you drive with your windows down in the summer? stereo louder? a/c on?

also, check your tire pressure. that is a large factor in mileage when the seasons change.

I'll check my tire pressure...what do you guys recomend for milage? I've been running the factory recomended pressure from the gas flap. I've had my windows down about the same as before, or up more to try to conserve fuel and I haven't used the AC (doesn't work anyways). There's really not been any driving habit changes at all.

Every time I "fill up," I usually put exactly 10 gallons in the tank. It's stupid, but I like to make mile-to-gallon calculations on the fly, and this makes it very easy.

A couple weeks ago, my 10-gallon tank went for about 150ish miles. Not very good, but I was driving rather hard often that week. The week after that, it rained hard for about 4 days in a row. All of a sudden, I got about 240 miles out of the tank.

I think how you are driving has a LOT to do with it.

I'd like to know how to get 32mpg on these cars! I usually get 25, or 27 on a good day. It doesn't really seem to change depending on how hard I drive it, most of my miles are highway miles anyway.

That's a myth. The tanks at gas stations are well burried and do not experience much temberature fluctuation at all.

As for winter and summer:

- "Winter" gasoline in many areas also contains additives that lower its freezing point to avoid fuel-line freezing, these additives decrease the energy density of the gasoline, causing more to be injected to produce an equivalent amount of power.

- The air is colder in the winter and colder air is more dense, this means more oxygen per unit volume and modern EFI systems respond to this extra oxygen by injecting additional fuel. You get more power, but use more fuel.

- In some areas "summer" gasoline is oxygenated (as has been mentioned). While this does yield lower performance than non-oxygenated fuel, it is generally nowhere near the loss from winter formulations (assuming you live somewhere that uses a large amount of anti-freeze compounds in the gasoline, this may not hold for places like Arizona).

- In winter many people let their cars idle to warm up, but they don't take that time into account when they calculate mileage, so that lowers the overall mileage perception as well.

I just saw on the news last week that you can get more gas in the tank when it's cold and it was recommended that you fill up in the morning or late at night as fuel expands as the temperature rises.

The news also stated that Canada has strict regulation regarding temperature of underground gas tanks but the US currently does not.

Although I'm not a scientist, this seems to make sense to me, especially since I live in the central valley of California where it can get 110 degrees in July and August.

Vern

My Honda has a noticable mileage difference in the winter. I'm told that the computer increases the fuel ratio, richer, in the winter. By the dealer. Not sure if other cars are the same.

The problem with that line of reasoning is that if you filled up your tank "to the brim" in the morning, and the temps got high enough during the day, the fuel expanding in the tank would leak if you had a vented cap, or potentially cause other damage. I know vapor in the tank expands and contracts substantially, but I can't imagine liquids don't do the same to a lesser extent.

Your reasoning sounds correct. I suspect that is why they don't want you to top-off the tank - because what expands finds its way out of the tank and into the atmosphere. However, if you fill your tank to a normal level (not topped off) and you by it at the coldest point of the day (or at least a few hours later after the tanks have cooled as well), you might be able to get more gas in the tank than when gas was warm and expanded.

It doesn't really matter that much with me cause I can't seem to drive my 944 in an efficient way - it's just too fun to drive fast.

Vern

The news is wrong. The news up here recommended that people buy Premium gasoline from the cheapest no-name station regardless of what their car requires because "higher octane is always better, and the no-name gas stations aren't any different from the name brands". Obviously the evening news is not a reliable souce for technical or scientific information.

The tanks are far enough underground that there's no significant change in temperature.

Ever gone into a root cellar in the middle of summer when it's ~100 out? It'll be ~60 degrees in the root cellar, and a root cellar isn't even completely burried. Put something 10-15 feet underground like a gas station's tanks are and you're talking about maybe, maybe a 15 degree swing in temperature from the coldest time in winter to the hottest time in summer. Daily fluctuation would be less than 1 degree.

For comparison, using measurements from a root cellar in Alberta, outside temperatures were less than -40 Fahrenheit and inside the root cellar the temperature was around 35 degrees (i.e. above freezing) but in the summer when temperatures were in the 90's the root cellar's temperature was around 50 degrees.

A 30 degree increase in temperature (from 60 degrees to 90 degrees) will cause an increase in volume of gasoline of 4 cubic inches. (From 231 cubic inches to 235 cubic inches.) Half of that increase, which represents the temperature change at tank depth equivalent to an above-ground swing of a staggering 130 degrees (remember that in the above example the below-ground temp changed from 35 degrees to 50 degrees when the above-ground temp changed from -40 to +90), would mean a change in volume from 231 cubic inches to 233 cubic inches. That's a change of 0.8% (0.008gallons) in volume from the coldest day of winter to the hottest day of summer. Change during the average day will be at most a single degree of fluctuation. Not enough to make any measureable difference.

The basic laws of physics and nature are at work here. The myth of filling up in the morning is persistant and will probably never die, but it remains a myth.