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I drive a 2001 Chevy Impala 3.4, and the car has 149487 miles on it. I drive five days a week round trip,from 48507 to 48067 about 117 miles round trip. I filled my tank up yesterday, and today after driving to and from work, I'm about one gallon from half a tank. I drive on 75 about 65-75 m.p.h I spend about $90 a week in gas. My car has no code ( engine). I already had a tune up about one month ago. Is this normal for a car to be a half tank with only one day of driving?
Supposedly I'm getting 22 mpg ( according to the digital computer on the car).
My car can hold 17 gallons I use about 6 going back and forth to work each day.
22mpg x 6 gallons of gas = 132 miles worth of gas. So it's not that far off the 117 mile round trip, especially if you make any detours (trips to stores, etc.), and with mpg depending on the type of drive (going uphill, stop and go traffic, stopping at lights, etc.). So based on the numbers you've given, that seems about right.
But like other people said, your gas mileage could actually be worse or better.
Also, 6 gallons used per day is more like 1/3 of your 17 gallon tank, not "one gallon from half a tank."
Keep in mind that when the fuel gauge shows 3/4, 1/2, 1/4, these are not all that accurate. So if the tank holds 20 gallons, you can't assume when you hit 1/2 tank that you have burnt 10 gallons and have 10 left. Could be off a gallon or 2 either way, not nearly accurate enough to figure MPG. I wouldn't base MPG on 1/8 tank either, it's too hard to get the car to the same level of "full" even if you are consistent with the gas pump nozzle (for example, put it on the 3rd click, let it fill, when it trips off the first time, you are done.) You will get cleaner data if you record several tank fulls where you burn down to 1/4 tank or so and fill back up. You can calculate the standard deviation on your data and figure out a reasonable "plus of minus" in terms of what you really measured.
No matter how you do it, just go by the gallons that are put in, the gauge is meaningless. I have had a car that took 90 miles to move the first 1/8 inch, then flew by the entire 3/4 in the middle in 90 miles, then took another 90 to get to the E line. My wife's Jeep, when you drive it below the E Line, so far that there is a needle width of space between the needle and the line, and there are still a bit more than 2 gallons in there, which is 10% of the tank capcity. Even car to car of the same make and model there can be variance.
It can be as simple as just calculating each time, or you can use a smartphone app to do it quickly and easily. I use one called Road Trip on my iPhone, which allows multiple vehicles and will track maintenance and allows you to set sub periods or specific trips even. That way you have a nice graph that shows over time. You also can know exactly how much it costs you per mile.
Interesting to see the first few tanks how the mileage went up as the engine broke in.
You will never be any more accurate than the computer...
You'll never be any more accurate than the data that is entered into the computer. You can 't get a good picture on just one gas fill-up, because there can be a significant difference in how "full" the tank will be after each fill-up, and there are likely to be events in a single tankfull of driving that will cause one trip to vary from another one. There is certainly no reliability to whatever figure you get in a trip of 117 miles, which can vary widely depending on factors like whether you had to warm up a cold car at the start, or not.
As for exact accuracy, if you are going to round off your mpg to the nearest mile, there is no point telling your computer to divide 331.86 miles by 15.08 gallons. Divide 330 by 15, which any fool can do in his head in 10 seconds. Or, just multiply your gas fill-up by 20. How easy is that? (Hint: double it and add a zero.) Compare the result with the miles you drive, and you will find how close you came to 20 mpg, which is really all you want to know. If you bought 15 gallons and went way over 300 miles, your mpg is way over 20, If you went way less than 300 miles, your mpg is way less than 20.
Last edited by jtur88; 01-15-2014 at 08:57 AM..
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