Gas mileage variances
 

I've found that my gas mileage varies from one tankful to another, with the variance between tankfuls being as great as 5 miles. Not sure what is causing this. Hence I am turning to brighter minds than my own asking for help understanding what is going on.

My driving habits are fairly consistant.
I wait until my tank is a quarter or a bit less before completely refilling. Sometimes at first, the gas mileage will be the same on the new tank , as it was before the new gas was put in... but then it changes and remains consistant within 1-2 miles until the next refill. Its almost like the last batch of gas has to run thru before the new kicks in. I always buy the same octane... 87.

The variables are that I buy gas from various stations and brands based on need/convenience and price. I've noticed worse gas mileage with a new tank after there have been several days of heavy rains. Better gas mileage after an oil change. Randomly worse and better gas mileage from gas obtained at the same station.

By far my mileage is superior in summer over winter. My fit does not like cold weather.

Has anyone else noticed this with their own vehicle?

thx
: )
Mzzchief

No no no. You cannot compare mileage between tanks. You do not always fill to to the exact level. Do it over a few tanks. In my case my milage is 32 over the first year.

Anyhow our mile is great so why both really?

okay. Well... my Fit is equipped with a little gizmo that shows "real time" gas mileage, that's how I know that there is a difference between tanks of gasoline.

To answer your question, the reason I am asking about the mileage, is because my suspicion is that some of the stations I frequent may be selling gas that for whatever reason is not as good as it could be. And that that gas could be harming the engine, with the result being poor gas mileage.

Additionally, I like to understand why things are as they are and I've found the more opinions I get, the easier it is to formulate an opinion that is closer to the truth. The "more heads is better than one" concept.

: )
Mzzchief

And the gauge is about as accurate as...well, it's not very accurate. Even after the computer reflash, mine is still always showing things as being high by 1-3 mpg compared to doing a calculation by hand.

Lots of stuff can affect your mileage, and lots of stuff can affect the accuracy of calculating it tank-by-tank.

If you go to station A, drive 300 miles or so, then go fill up at station B and calculate that you got 35 mpg based on the amount you put in, and then go back to station A again after 300 mile and calculate 31 mpg, it's probably because the pumps are just cutting off the fill level at a different spot; you can't necessarily conclude that station A has "better gas" from a test like that. Even if you used the same station and the same pump each time, you'd still have this issue.

Weather can affect mileage a lot too. Temperature, humidity, and especially wind can all have an effect on fuel consumption. And while you might think that your driving conditions stay relatively the same all the time, little changes (like one bad traffic jam, or a day or two of quicker short shopping trips) can change consumption too.

In short: don't worry about it. Drive efficiently, and you'll get fine mileage in the end. Overreacting to a calculation or an untrustworthy gauge one one tank isn't necessary.

Summer and winter gas mixtures are different, with the summer formulation having a higher energy density.

You could also be seeing variations in ethanol levels. Ethanol has a lower energy density as well, so a 10% ethanol mixture will end up with a lower milage than 0% ethanol.

Well said. I was not inclined to be wordy. I like using the gas gauge myself. When it is between 1/4 and 1/2 I fill up. Screw the milage.

little gizmo that shows "real time" gas mileage - it is a gizmo and not worth looking at as it is inaccurate until it is reflashed and even then soso.

Thx.

The general consensus seems as if the problem is more one of the realtime guage and nothing to do with the actual gas mileage from one tank to the next.

That said, has anyone found one brand of gas to consistantly out perform another?

: )
Mzzchief

You do clear the trip mileage when filling up, don't you? If not, at each fill up clear the trip mileage, which will also clear the average mileage readout by default. Then go to the average mileage readout to keep track of over the course of the tank. If you're doing this, then disregard. It didn't seem clear that you are. And you'll see some wild averages from low to high for a short time but by about half a tank it will settle in closer to your actual average as the tank is being consumed. Depending on how and where you're driving over the course of the tank [among other things] you'll see some differences from fillup to fillup.

Dan

It all depends on where you live and which refineries your stations are getting their gas from again based on geography. Either way, your best bet is to find a brand that seems to do better in your experience and stick with it where ever possible. Jumping brands back and forth will never give you consistency due to their minute variations in their formulations.

Having said all that, I personally use Shell in all my vehicles whenever possible since I noticed a better and consistent mileage readings with them at least in my area.

Don't let tank get to empty, The more air in tank the more vapor(gas) goes back into station tanks. Try to use same gas from same station. Cheaper is not always better. cheaper gas might have more ethanol. Use a top tier gas. If your mileage still fluctuates, try using a higher octane. Higher octane is more constant and could give better mileage. I averaged 37 mpg in 3 years with winter in Chicago averaging 33-35mpg using prmium and M1 oil. Also fill up in morning, I filled up once in 90degree day an put 12.5 gallons in a 13.2 gallon tank when I still had almost a quarter tank. Told the gas station owner and gas co. and they did nothing. Check the oil and go with maintenance minder unless you have to add oil because oil broke down. In the high heat I would use a synthetic oil, and wouldn't change brands. Find an oil and use it. Every oil had it own chemical make up and constantly changing oil brands might clean the protection and dry out seals from chemical reaction, have had that problem.