Mpg reading and partial fill ups

Discussion in 'Fuel Economy' started by Ec031299, Aug 23, 2016.

  1. Ec031299

    Ec031299 Member

    hello, first, I am new to this forum. Seems like a great resource!
    I am looking into getting an obd2 scanner such as kiwi, ultraguage,scanguage, etc.
    My main question is this: I am a teen and I only get so much money for gas each week, meaning that I don't ever have enough to fill my tank up. I do know that if you do not enter a tank size on whatever device you are using that it still gives an mpg rating. How far off is this reading? I have heard up to 10%. Now if I enter my tank size, and do not fill my tank up, will this change the reading at all? What have you guys experienced with partial fill ups and the accuracy of mpg readings?
     
    BillLin likes this.
  2. Welcome to the site!

    What are you driving?

    Sorry, no help on the partial fill ups, but a free bump
     
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  3. Ec031299

    Ec031299 Member

    97 Tahoe 4dr 4wd
     
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  4. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I "think" , that when you get your gauge dialed-in , partial fillups won't affect anything. My SG was not 100% accurate out of the box. It took several (5+) fillups to calibrate it. But there may be other ways I'm not aware of to do this.
     
    BillLin likes this.
  5. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Even if you don't have the gauge calibrated, any relative mileage changes will be accurate. If you improve from 15 mpg to 16, that's a 6% improvement, even if the actual numbers were 16 and 17, or 14 and 15. Don't sweat the precision too much.

    Most OBD gauges read a mpg little high at first, so you can pre-calibrate it to be close. The easiest way is to set the engine bigger than it actually is. You probably have a 5.7L V8, so try setting it to 6.0.
     
    BillLin likes this.
  6. Ec031299

    Ec031299 Member

    Ok, and if I can get the tank filled up with it set to 6.0, will the reading be accurate?
    Thanks
     
    BillLin likes this.
  7. BillLin

    BillLin MASS: 2018 Bolt EV and 2017 Prime

    [note that some good info came in while I was writing this...]

    Welcome! You're off to a great start giving thought to your situation and how to make the most of what you have.

    I suspect the slight inaccuracy of the measuring device (within 10%) is less import on a high fuel consumption vehicle than on a very low fuel consumption vehicle. Hope for precision and consistency in the measurements. You'll see numerically greater MPG difference in the more efficient vehicles. That's not to say a 1 MPG improvement for you is insignificant. Quite the opposite. Try to take the gauge numbers as a guide to make relative improvements. Your true improvements will show when you are eventually able to do tank to tank fillup comparisons, or your weekly partial fills take you farther with similar driving conditions. You'll get a feel for how the fuel gauge needle drops and you'll see improvements there as well.

    Air up the tires, try not to make short trips, and try to combine errands into a single trip. The warmup cycle has a high impact on your overall fuel efficiency. Remember that you, yourself, are the greatest tool for improving your vehicle's fuel efficiency. Drive with a gentle foot and a peaceful mind. Learn the routes you take (traffic patterns at different times of the day, traffic lights, etc.) and find better alternative routes. Keep your highway speeds down if you are able. Have a look at the steady state speed vs. MPG charts that Wayne Gerdes has produced for many vehicles. You can learn from the graph for almost any vehicle.

    These are just a few tips that come to mind based on your introduction and vehicle. Keep up with your vehicle maintenance, especially oil changes. I'm sure you'll receive many good suggestions if you ask specific questions.

    Take care,
    Bill

    The engine size setting affects the calculated/estimated rate of consumption of fuel. For fillups, you provide the actual amount of fuel you pumped, assuming you are trying to calibrate. This step is optional if your gauge is already calibrated or you are satisfied to leave the settings as they are. If I recall correctly on my SG II, changing the engine size does affect "current" measurements. Someone please correct me if I'm wrong. It's been a while since I looked at these aspects... Thanks.
     
    PaleMelanesian likes this.
  8. Ec031299

    Ec031299 Member

    Thanks for the great reply. I have been feather footing it for a long time already and have been coasting as much as possible. I did have the opportunity to top off to a full tank and I got 16.4 mpg combined, which is 17% more than the epa combined rating (14). That driving was comprised of city/rural country rd driving at 55. Tires are aired up to max recommended pressure.
    Ps: my tank is 30 gallons!
     
    BillLin likes this.
  9. BillLin

    BillLin MASS: 2018 Bolt EV and 2017 Prime

    Good job! Now if only my brother-in-law would drive his Tahoe in similar manner... :)
     
  10. marydaughterofdodge

    marydaughterofdodge New Member

    On another forum from 2010 a couple people mentioned having a hard time finding pure gas. What is pure gas?
     
  11. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Yeah, it's pretty much impossible to find now. Most gas you can buy is 10% ethanol, 90% gasoline, called E10. It's marked as such on the pump. Ethanol has less energy per gallon than gas, so E10 gives you lower mpg. Different cars handle it differently, but on average it's about 3% lower mpg with E10.
     
  12. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    Some people may dispute that 3% figure. Jay ? I have no access to E0 , so I don't know. My gut tells me 5-7%. If you can find it without going to the next state , try it out. Document your results, and see if the price differential is worth it.

    But Mary, Andrew is VERY knowledgeable about all things hypermiling. You can trust his advice. I'm just offering an opinion.
     
  13. marydaughterofdodge

    marydaughterofdodge New Member

     
  14. marydaughterofdodge

    marydaughterofdodge New Member

    Ah yes, now I remember, YEARS ago they started adding the ethanol to gas. Didn't realize there was any of the "pure" gas around anymore. Thanks for the info.
     
  15. Pure-gas.org
     
  16. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    I found one in Kansas last month on vacation, but it was $0.40 more than E10. That's not worth my money for a minor mpg gain.

    Ethanol has 67% the heat energy of gasoline, so E10 has 3.3% less energy than pure gas. You would see exactly that difference if you run a furnace on the two fuels. A car engine is somewhat different because of compression and octane, with ethanol being much higher octane. Depending on how the engine is tuned it may change how it handles E10. Back when it was first introduced I could test at different stations with and without. Doing my best to adjust the results for other variables (city/hwy, temperature, passenger loading, etc) I came in right at the expected 3% difference. Add a bunch of *** asterisks because it's a small uncontrolled sample, but it matches the math.
     

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