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265 to 285 Mileage/MPG Error Calc

Discussion in 'Wheels & Tires' started by Relik, Aug 1, 2011.

  1. Aug 1, 2011 at 2:04 PM
    #1
    Relik

    Relik [OP] Well-Known Member

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    Hey all,

    I'm about to make the jump myself to either 275 or 285 and I'm interested in BFG All terrains. As many I am afraid of mpg loss so I compared all the numbers and here is the mileage error I have calculated.

    After MUCH research on here, I think people forget their mileage is off, hence why the MPG loss is complained about/why some see big drop in mpg. The tire is bigger, achieves more distance in same amount of rotations hence why mileage is portrayed less than actual=mpg off.

    From my calcs the switch leaves you at about -2MPG error just from the circumeference difference
    (Dunlop GrandTek AT20 265/65/17 to 285/70/17 BFGs).

    Now the weight also adds mpg loss ofcourse, but now it seems considerably less if the geometric loss noticed is only perception....for ex. if the ones who said they dropped to 15mpg after switch (from 18-20mpg previously), it was actaully 17mpg....etc....

    Thoughts?

    Tire_Size_MPG_Error.jpg
     
  2. Aug 2, 2011 at 11:51 AM
    #2
    Relik

    Relik [OP] Well-Known Member

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    No thoughts? :D
     
  3. Aug 2, 2011 at 12:10 PM
    #3
    gov875

    gov875 WahooTaco

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    how does this factor in with my current speedo reading high? right now i'm 1-2 high around 30 mph and around highway speeds it gets as much as 4 off. This i've seen vs GPS's and radar signs so i doubt they're all mis-calibrated like my speedo is. Not sure if it ties into the odometer or not.

    i'm about to switch to 285s so i sure hope you're right since i'm getting about 18-20 with my normal drivin now. good work on the calculations man.
     
  4. Aug 2, 2011 at 12:19 PM
    #4
    AuburnTaco

    AuburnTaco Active Member

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    I recently upgraded form the stock 245's to some general grabber AT2's at 285/75R16. I went to the dealership and had them change the odometer/speedometer to accuractely reflect the larger tire size. I was getting about 21 mpg with the 245's and now i'm getting about 16 mpg with the 285's. I have no idea how accurate my speedo actually is.

    You need to factor in for frictional losses as well.

    Also, I have quite the lead foot and I sit in stop and go traffic which might not be a good comparison for the change in mpg
     
  5. Aug 2, 2011 at 12:26 PM
    #5
    gov875

    gov875 WahooTaco

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    i knew i could go to the dealership to get the speedo adjusted for the bigger tires but since it came from the factory reading low for the stock tires i figured that's pretty much a lost cause. some people have said their speedo actually starts to be about dead on with the change from 265 to 285s.

    i might have to get a scangauge or ultragauge to show me my MPGs just to make sure i don't get a lead foot
     
  6. Aug 2, 2011 at 12:26 PM
    #6
    mxnborder

    mxnborder Well-Known Member

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    nice writeup where did you find that chart deal? i always wonder what my actual mpg is i usually calculate about 17ish with my 285's but know its off.
     
  7. Aug 2, 2011 at 12:45 PM
    #7
    macgyver

    macgyver Well-Known Member

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    My speedo is about 5mph at highway speeds (70ish)
     
  8. Aug 2, 2011 at 1:22 PM
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    Relik

    Relik [OP] Well-Known Member

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    I calculated it all using data from tire rack, comparing the difference in revolutions per mile of each tire to stock.
     
  9. Aug 2, 2011 at 1:24 PM
    #9
    Pugga

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

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  10. Aug 2, 2011 at 1:32 PM
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    Juggernaut

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    That doesn't take into account rotational inertia, which is the big player here. Not only is the tire heavier, but now the radius is larger.
     
  11. Aug 2, 2011 at 4:57 PM
    #11
    Tacomadude89

    Tacomadude89 Well-Known Member

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    Yeah, I agree, that is definitely true in a lot of cases.
     
  12. Aug 8, 2011 at 6:56 PM
    #12
    jgang

    jgang Well-Known Member

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    Use this. Plug in factory size, then aftermarket size. The difference in diameter will give you the factor to multiply your odometer reading by when you check your gas mileage. I.e., factory 265/65/17 vs aftermarket 265/70/17 is a difference of + 3.4%. Multiply that (1.034) by your odometer reading and you get your actual mileage traveled. Double check it with the figures on the site....divide the actual reading by the speedo reading and you'll get 1.034. I've verified this formula with my GPS a dozen times and it's spot on. No need to mess with "rotational weight" as it's a non-factor in calculating actual road mileage. It's important if you're just running a bunch of figures for kicks, but for real world calculation of distance traveled, it's irrelevant. A mile traveled is a mile traveled, whether it's on "heavy" 285s or "lighter" 265s. The rotational weight may effect your fuel mileage, but has no bearing on a distance traveled calculation when switching tire sizes.

    Love that Excel chart OP!
     
  13. Sep 14, 2011 at 3:59 PM
    #13
    Srpat

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    Figure this is as good a place to ask as any. 285's will cause the engine to have to work harder with the extra resistance, but with my 07 TRD 6 speed manual, would I increase highway mpg by increasing tire size?

    Those of you with this tranny know that the sixth gear is kinda high on the manuals...that's why the autos get a 1 mpg better rating than we do. Having taller tires would be like regearing the tranny, wouldn't it? Anyone see this that went with 285s?
     
  14. Sep 14, 2011 at 4:39 PM
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    Tacomadude89

    Tacomadude89 Well-Known Member

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    In a lot of cases, yes, the larger tires can cause a little better mpg on the highway due to the lower rpm's. A lot of people have reported that.

    Of course, many other factors come into play such as a headwind or climbing a grade which change everything.
     
  15. Sep 14, 2011 at 5:48 PM
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    mxnborder

    mxnborder Well-Known Member

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    lets all put 37's on our tacos :quickdraw:
     
  16. Mar 24, 2012 at 10:03 AM
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    xgalvanxgx

    xgalvanxgx seymour butts

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    hey Eddy, i just switched to 285/70/17. whats my average mpgs now?
     
  17. Oct 4, 2016 at 8:39 PM
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    kjkroll83

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    I knew this is really old but worth a shot. If the difference in tires is 3.4% where did you get 1.034? The difference from my 265/75/16s vs stock is 1.63 but that would give me 27mpg. Which is obviously wrong. Talking to myself lol
     
  18. Oct 23, 2016 at 8:33 AM
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    Tacoma_Tri

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    3.4% /100 + 1 = 1.034
     
  19. Oct 23, 2016 at 8:35 AM
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    kjkroll83

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  20. Oct 23, 2016 at 8:40 AM
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    AdventureKid

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