Quote:
Originally Posted by Guildenstern
Chevron and Shell are Top Tier Detergent gasoline, the stuff BMW recommends http://www.toptiergas.com/
While it's true all the gas comes from the same refinery distribution. What shell or Chevron does with it before it ends up in your tank, vs what costco or GetGo do is a difrent story.
Cheap gas usualy has little to no cleaning additives, may not be filtered or dried as thoroughly prior to showing up at the station, and most importantly cheap gas is "watered" down with the maximum government allowance of ethanol which is basically the High Fructose Corn Syrup of gasoline. (which coincidentally comes from corn!)
Ethanol retains water (un-like petrol it's water soluable) so it causes corrosion and damage to seals designed to only deal with petro products, the water can come out of solution due to temperature change and as the ethanol percentage changes and leave a puddle of water in your tank, and overall ethanol has lower energy. 1 gal of Ethanol has less energy in it than one gallon of un-blended gasoline. Gasoline has about 33.41 kWh per gallon of energy, whereas DRY ethanol has only 22.27 kWh per Gallon (it has even less energy per volume when there's water in solution with it.) At the federal max of reformulated ethanol blend (the cheap stuff *looking at you sunoco*) you end up with a liquid that only has 98.14% of the total energy of pure gasoline. And that's before you take into account the variations caused by different specific heats and how that effects engine efficiency etc... http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent
Lower energy, and adverse engine efficiency properties means you need to burn more fuel to get the same performance. You may not notice it, but even to maintain freeway speed you are a little deeper on the throttle, and the engine is running a little richer. That adds up to lower mileage.
SCIENCE!!  |
The blend stocks at the refinery are the same, so the level of filtering and drying done there is consistent across brands.
The level of maintenance on the filters and separators at the station will differ wildly.
All gas is mandated to have cleaning adds. This is an EPA requirement. Top tier will have more of them.
Top tier gas will also have 10% ethanol by its specification. There is no such thing as gas that is watered down with ethanol. While in some areas, ethanol free gas is available, for one thing it cannot be top tier by specification, and for another it is usually from no-name stations so that goes back to the earlier points. Id buy ethanol free if I could, and I run my own additization.
The i series does indeed have a small tank, but at least the N55 135i cars are very easy (in MT form at least) to yield great MPGs. I have routinely gotten 32 MPG tanks when doing highway driving.