Trip computers are the most reliable way to track fuel economy
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2008-2013 Cadillac CTS General Discussion Discussion, Trip computers are the most reliable way to track fuel economy in Cadillac CTS Second Generation Forum - 2008-2013; At least that's what this article claims. Thought it would be an interesting thread for debate considering all the gas ...
  1. #1
    gohawks63's Avatar
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    At least that's what this article claims.

    Thought it would be an interesting thread for debate considering all the gas mileage talk. http://www.freep.com/article/2014020...-trip-computer

    With the V, I just make sure I have enough to get to the next gas station.
    Sailirish7 likes this.

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    billeee is offline Cadillac Owners Fanatic
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    Re: Trip computers are the most reliable way to track fuel economy

    Trip computers don't directly measure fuel flow rate. They calculate fuel flow rate based on engine speed and the pulse width of the injector opening signal. This assumes that all vehicles have injectors that flow at he same rate, have the same opening and closing speed, and the fuel pressure is exactly the same for every car.

    If trip computers were more accurate, the EPA and auto magazines would be using them to establish fuel mileage. The last I heard, they were still using the fill - drive - fill method.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by gohawks63 View Post
    With the V, I just make sure I have enough to get to the next gas station.
    That was always my Corvette's problem on road trips through the Dakotas and Montana. I actually ended up sleeping in it at a gas station on one trip, waiting for the station to open up.

    The problem really is the size of the tank. I have owned Honda Accords with bigger tanks. If you can't have mileage, it would be nice to have capacity.
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    Re: Trip computers are the most reliable way to track fuel economy

    I think the computerized calculation is usually more accurate than trying to determine MPG manually. The real problem with calculating by hand is that the amount of fuel supposedly used is always determined by how much is put in the tank to replace it. That can vary considerably due to several factors: the angle the car is on (it may hold more or less fuel depending on how much air is trapped in the tank due to the way the car is resting -- tilted left or right, up or down); how high you fill it (to the point the pump shuts off or until it's starting to spill over); and the accuracy of the pump's meter (note that in most states gas pump meters are checked by the bureau of weights and measures or whatever only once per year). Those variables make short-term calculations particularly suspect, although they tend to be less significant in longer tests (i.e., several tanks over 1000 miles or more). However, despite persistent urban legend to the contrary, temperature of the fuel is not a significant factor -- the amount of expansion or contraction of liquids due to temperature is negligible.
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    Re: Trip computers are the most reliable way to track fuel economy

    "Gas station pumps are usually accurate — they’re regulated by the states — about how much fuel they deliver, but it’s almost impossible for the average driver to be sure the tank is equally full from one stop to another. That’s because of variables such as temperature, sloping gas station parking lots and pumps’ flow rates".

    Then they state “It’s very unrealistic to measure fuel consumption based on the pump. Even the same pump can be way off as to what constitutes a full tank from one stop to the next.”

    I'm not sure what they are trying to say...that we are too dumb to take the total number of gallons consumed and with the total number of miles driven we can't figure out what our mpg is?

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    Re: Trip computers are the most reliable way to track fuel economy

    I'm not interested in fuel economy to the degree where I want to go back to keeping receipts and logging distances travelled on them. Having had a trip computer in three of my previous vehicles, I am comfortable with GM trip computer calculations telling me whether I have the fuel range to reach the particular exit that I want to stop for food and fuel, or whether I need to get fuel sooner.

    Once I get familiar with the fuel mileage my vehicle delivers, I usually monitor fuel range and tire pressures, and occasionally % oil life remaining. With a new car, IMO the mileage is what it is until I get another car, and tune ups won't help. The last vehicle that I had tuned up was my 95 Lumina APV, which I thought had platinum plugs, but didn't. I ran the standard factory plugs past 80,000 miles before it coughed to tell me it needed a tune up. I just blow the dirt out of the air filter and keep on driving.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by tinman View Post
    ...I'm not sure what they are trying to say...that we are too dumb to take the total number of gallons consumed and with the total number of miles driven we can't figure out what our mpg is?
    They're saying what I said -- the data from gas-station pumps are unreliable. As any scientist or engineer or statistician will tell you, GIGO (garbage in, garbage out).

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    Re: Trip computers are the most reliable way to track fuel economy

    The standard accuracy test run by states on gas pumps allows a maximum error of 0.5 %

    Here's an article on trip computer inaccuracy

    http://www.edmunds.com/fuel-economy/...s-fibbing.html
    928S and 99flhr like this.

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    Re: Trip computers are the most reliable way to track fuel economy

    True, but in this jurisdiction the pumps are tested only once per year, and it's not unusual that they're found to be out of compliance.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by hauler View Post
    With a new car, IMO the mileage is what it is until I get another car,
    +1 IMO the absolute accuracy of either method is relatively unimportant, the only reason to monitor it occasionally is that a substantial decrease could serve as a heads up for an unnoticed mechanical problem. I`m just looking for consistancy.

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    Re: Trip computers are the most reliable way to track fuel economy

    Thanks, billeee.

    Your linked Edmunds investigative report, including mfrs' responses, thoroughly refutes the Detroit Free Press filler-fluff piece sourced from co-dependent auto industry mimic WardsAuto.

    When I need to know miles per gallon, I'll reach for pen, paper, and/or my trusty ol' HP 12C handheld calculator.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by The_Judge View Post
    True, but in this jurisdiction the pumps are tested only once per year, and it's not unusual that they're found to be out of compliance.
    Here's a study on pump accuracy in Michigan. Less than 2% of pumps were beyond the 0.5 percent accuracy.

    http://www.mlive.com/business/index....dit_weigh.html

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    Sailirish7 is offline Cadillac Owners Member
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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by The_Judge View Post
    I think the computerized calculation is usually more accurate than trying to determine MPG manually. The real problem with calculating by hand is that the amount of fuel supposedly used is always determined by how much is put in the tank to replace it. That can vary considerably due to several factors: the angle the car is on (it may hold more or less fuel depending on how much air is trapped in the tank due to the way the car is resting -- tilted left or right, up or down); how high you fill it (to the point the pump shuts off or until it's starting to spill over); and the accuracy of the pump's meter (note that in most states gas pump meters are checked by the bureau of weights and measures or whatever only once per year). Those variables make short-term calculations particularly suspect, although they tend to be less significant in longer tests (i.e., several tanks over 1000 miles or more). However, despite persistent urban legend to the contrary, temperature of the fuel is not a significant factor -- the amount of expansion or contraction of liquids due to temperature is negligible.
    Then how do you account for the fluctuation of the energy content as the fuel expands/contracts due to temperature? Not trying to be funny, but this must have an effect even if it is negligible.
    hauler likes this.

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    Re: Trip computers are the most reliable way to track fuel economy

    I did a 2K mile trip in mid December. I zeroed the computer and kept all the gas receipts. At the end the computer and the receipts were within 1 MPG. Close enough that I will never collect gas receipt again

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by hauler View Post
    That was always my Corvette's problem on road trips through the Dakotas and Montana. I actually ended up sleeping in it at a gas station on one trip, waiting for the station to open up.

    The problem really is the size of the tank. I have owned Honda Accords with bigger tanks. If you can't have mileage, it would be nice to have capacity.
    I completely agree. A 17 gallon tank is too small for a vehicle that gets between 12-17 mpg.

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