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Another MPG thread - Anyone account for tire size and odometer discrepancies

Discussion in '2nd Gen. Tacomas (2005-2015)' started by bgsmith, Jan 9, 2012.

  1. Jan 9, 2012 at 6:38 PM
    #1
    bgsmith

    bgsmith [OP] Well-Known Member

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    OK, so I religiously track my MPGs, since I commute about 100 miles a day it has become almost a hobby to me.

    Well I have 265/70 17s on my truck which resulted in about a 3.5% (conservatively calculated) drop in my odometer. My commute went from about 47.5 miles to about 45.5 miles, does anyone account for this difference in calculating gas mileage?

    I recently got 19.7 MPG on my last tank, 290 miles for 14.699 gallons. Taking into account the change in odometer you could say I actually traveled about 300 miles which would result in my MPGs being 20.4.

    Thoughts?
     
  2. Jan 9, 2012 at 6:41 PM
    #2
    TACWUT

    TACWUT Well-Known Member

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    I have dealing with that too but I'm a dumb ass who cant track mpg lol.
     
  3. Jan 9, 2012 at 6:44 PM
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    Pugga

    Pugga Pasti-Dip Free 1983 - 2015... It was a good run

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    I always correct for the larger tires. The 3.5% is pretty damn close to a correction factor and the one I use for correction for that size tires also. I'm close to your MPG also, a little less now that it's getting cold out.
     
  4. Jan 9, 2012 at 6:49 PM
    #4
    joes06tacoma

    joes06tacoma Well-Known Member

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    My odometer is accurate, but I have adjusted for oversized tires on previous trucks. If you don't account for the difference, then you're not being accurate and you'll just bitch about your MPG that much more.
     
  5. Jan 9, 2012 at 7:05 PM
    #5
    bgsmith

    bgsmith [OP] Well-Known Member

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    Just did a quick adjustment using 4% as an adjustment factor and I must say I am pretty happy with my MPGs.

    Average about 18.69 MPGs over 16 fill ups. Best is 20.56 MPGs, worst is 17.09 MPGs.

    The 4% adjustment factor is due to the fact that my commute was 47.5 miles one way to work and now my odometer is reading 45.5 miles each way.
     
  6. Jan 27, 2012 at 8:00 AM
    #6
    jgang

    jgang Well-Known Member

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    In running 265/70/17s, I multiply my odometer reading by 1.035 to get the actual miles traveled. IIRC, I cross-checked it with my GPS several years ago and found only a 2-3 mile difference between the GPS reading and the aforementioned formula. Good enough for me.
     
  7. Jan 27, 2012 at 8:02 AM
    #7
    chadderkdawg

    chadderkdawg Don't ask questions you don't want the answer to..

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    So what would you all use for a 33 inch conversion factor?
     
  8. Jan 27, 2012 at 8:18 AM
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    Pugga

    Pugga Pasti-Dip Free 1983 - 2015... It was a good run

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    Use this:
    http://www.tacomaworld.com/forum/tirecalc.php

    The Revolutions per Mile is given in a percentage from old tire size to new. Multiply your trip ODO reading by that percentage to get actual miles traveled.

    Ex. the difference in old tire size to new for me gave me 3.6%

    Trip ODO x 1.036 = Actual miles traveled.

    I just plugged in 285/70R17 and it says 6.6%
     
  9. Jan 27, 2012 at 8:26 AM
    #9
    2TRunner

    2TRunner Don't give up here just yet

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    I did the math and then had the wife recheck it (math major) when I went from 215/70/15 to 225/75/15 on my 5 lugger. Ratio was 1:1.05. I figured mine out the ol' fashioned way, didn't think to figure it the easy way pugga posted.

    I've got 30's now, very fresh 30's, I think that ratio is now at 1:1.10 by the way I figured it (the long way)
     
  10. Jan 27, 2012 at 8:32 AM
    #10
    chadderkdawg

    chadderkdawg Don't ask questions you don't want the answer to..

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    Man I never took the time to notice the percentage change on that sheet... Went from 265/65/17 to 285/75/16's and its showing a 7.4% change.... Pretty significant... Tank is getting low, I'm anxious to apply the 1.074....
     
  11. Jan 27, 2012 at 8:41 AM
    #11
    Cowboyz

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    I have 35s on my truck and so my ODO is off by 11-12%. so i take my total miles, multiply by .12 then add that number to my total miles for that tank to get the total miles. then divide by your gallons. im getting like 15 in town and 17 on the freeway without regearing.
     
  12. Jan 27, 2012 at 8:44 AM
    #12
    chadderkdawg

    chadderkdawg Don't ask questions you don't want the answer to..

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    Just multiply your total miles by 1.12, that will eliminate the step of adding the miles back.
     
  13. Jan 27, 2012 at 8:46 AM
    #13
    Pugga

    Pugga Pasti-Dip Free 1983 - 2015... It was a good run

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    x2, no need for the added step :thumbsup:
     
  14. Jan 27, 2012 at 8:47 AM
    #14
    chadderkdawg

    chadderkdawg Don't ask questions you don't want the answer to..

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    My adjusted mileage is about 18.5, which leads me to believe my new tires didn't affect my mileage at all... w00t!
     
  15. Jan 27, 2012 at 9:05 AM
    #15
    jgang

    jgang Well-Known Member

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    That's suprising. If you have a GPS, I'd cross check it just to make sure. If you want to get precise, you can take your GPS mileage reading and divide it by your odometer reading to get you multiplication factor for future tanks.
     
  16. Jan 27, 2012 at 9:15 AM
    #16
    T@co_Pr3runn3r

    T@co_Pr3runn3r XXXXXXXXXXXXXXXXXXX

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    Here's my calculation issue....265/70/16s 30.6" diameter, which is a stock tire size. GPS says 68 - the speedo says 70. The trip/odo are spot on with the frwy mileage markers over a 20 mile range. WTF adjustment needs to occur? Which of the 3 things can I trust? Mileage and gallons used are the 2 main things but if there is a speed discrepancy then how can the mileage be correct? Usually filling with 17-18 gallons and trip is usually in the 360-390 range, mostly flat mostly frwy driving.


    Here's what's stupid...the 05 had 32.1" diameter tires and the speedo & GPS were completely in sync all the time but the mileage markers on the frwy were off by .1 to .15 each mile so about a mile or mile and a half off every 10 miles. Doesn't really add up either.
     
  17. Jan 27, 2012 at 11:16 AM
    #17
    jgang

    jgang Well-Known Member

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    If you're stock, I don't see any reason to not trust the odometer as-is. The truck was set up from the factory that way, so it should be good. Not sure what to make of the discrepancy between your GPS and odometer. :confused:

    I'm getting somewhere between 340 and 360 miles per tank with 265/70/17s.
     
  18. Jan 27, 2012 at 12:54 PM
    #18
    ItalynStylion

    ItalynStylion Sounds Gooooood

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    The real mind fuck is that the speedo/odometer isn't calibrated from the factory for the stock tires.

    My TRD sport usually read 3mph over the actual speed I was doing when I was going roughly 60mph. This inflation is also present in the odometer so it seems like you're going farther than you actually are.

    My take? Toyota probably did it for two reasons. If you divide your mileage by the fuel you put in it SEEMS like you're getting better gas mileage than you really are. Not only that, but you'll outrun your warranty quicker too since you'll be racking up the miles faster.

    I put larger tires on (Duratracs) and my speedometer reads dead on now. When I calculate my mileage I'm getting roughly 2-3mpg worse than I was before on stock tires. I don't think it has to do with the tires slowing me down or actually taking more fuel to turn but rather the actual change in what the odometer is reading due to the increased circumference of the tire. Long story short, you were getting crappy gas mileage all along you just didn't know it. ;)
     
  19. Jan 27, 2012 at 1:19 PM
    #19
    Pugga

    Pugga Pasti-Dip Free 1983 - 2015... It was a good run

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    The odometer is dead on the from the factory with stock sized tires (check it against mile markers or GPS). The speedometer reads fast from the factory. when you oversize your tires, you usually correct the speedometer and then throw the odometer out by a certain percentage (which is why you now need to correct if you want accurate MPG info). I found that by going to a 265/70R17 tire, my speedo is dead on with my GPS and also checked against several other vehicles but my odometer is reading less miles than I'm actually driving.
     
  20. Jan 27, 2012 at 1:37 PM
    #20
    ItalynStylion

    ItalynStylion Sounds Gooooood

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    Why would there be a differential between the two if they get info from the same source?
     
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