Catch me if you can (MPG)

Discussion in 'VP44 Pump' started by Mopar1973Man, Aug 28, 2011.

  1. Stangshcky12

    Stangshcky12 Super Moderator Staff Member

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    The speedometer is off by a percentage when increasing tire size. A 31-35 will cause the speedometer to be off about %10 which is why it is more noticeable at higher speeds.
    I'm just guessing here but to figure out the odometer difference you would use to divide a mile by circumference in ft. And get how many rev. your tire does and divide it by what it did
    (31"*pi)/12= 8.11583333 ft/rev
    5280/8.11583333= 650.580142 rev/mile
    (35"*pi)/12= 9.16 ft/rev
    9.162978577 = 576.231839 rev/mile
    That's about 12.902% or for ever 1 mile on your odometer, you went 1.12902 miles

    Orrr you could just divide new tire size by the old and you can get the difference.....Lol
     
    Last edited: Feb 9, 2012
  2. Rogan68

    Rogan68 Senior Member

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    My stock tire is 245/75-16. I've got 235/85-16s on it now. 4.1% offset.

    245/75-16:
    30.5" dia.
    7.98 ft/rev
    662 rev/mi

    235/85-16:
    31.7" dia
    8.29 ft/rev
    636 rev/mi


    662 / 636 = 1.0408, or 4.1% (rounding)

    60mph + 4.1% = 62.46, or 62.5mph.
     
  3. Mopar1973Man

    Mopar1973Man Senior Member

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    So how do you explain that I'm currently running a stock sized tire 265/75 R16 and my speedmeter is off by 0.5 MPH and total trip difference for a tank of fuel can vary 20-30 miles? GPS and ScanGauge II are right on but the odometer is off???

    As for the cetane grid I got it from a ASTM document... I had a connection with a guy that allowed me to grab it... But this is all I got left of that document... I lost the document in one of my hard drive crashes.
     
  4. Stangshcky12

    Stangshcky12 Super Moderator Staff Member

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    I might have to go to the library this weekend and see what I can find there as far as ASTM standards on this.

    How many miles a tank are you getting? The speedo/odometer gears in your truck might be for 245/75/16 (everywhere list that as the stock size, although I'm not doubting that 265/75/16 may have been an option) so if your using 25-30 gallons a tank at 19mpg and your odometer is off 3.73% your looking at 17.7 to 21.261 gallons a tank.
    This is of course all nominal numbers and we know tires are not always ideal size and average mpg is ever changing.

    As far as the speedometer, speedometers suck lol I dont really have much of an idea there. Ive driven countless vehicles from my cousins car dealerships and the speedometer was typically 3-5 mph faster then what the GPS read (which of course can be up to 1 mph off (older ones) newer ones seem to be .3-.4 mph off since they use several signals, not just four like old ones)
     
  5. Mopar1973Man

    Mopar1973Man Senior Member

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    Thank you... That's what I want someone else to voice right there... So if your looking for true number you got a use a device calibrated for your application. Hence my setup of the SGII and GPS.

    I know know that MPG's is just averaged numbers using the best information you can provide for the math. So if the information is skewed then so is the results.

    As for the tires that would be a 2.3 MPH change so I'm right on the tires just the stock speedo/odo is calibrated poorly.
     

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