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Calculating MPG with larger tires

Discussion in 'Wheels & Tires' started by Goosedog, Jul 12, 2013.

  1. Jul 12, 2013 at 4:14 PM
    #1
    Goosedog

    Goosedog [OP] Well-Known Member

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    Is there a chart here somewhere that will help me to get a ratio or divider to figure my milage with tires larger than stock?

    Thanks
     
  2. Jul 12, 2013 at 4:29 PM
    #2
    bolio

    bolio Well-Known Member

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    It's hard to calculate just on size, since the weight is also a factor. And even the same brand and size can have a 15 lb difference from a P to E rated tire.
     
  3. Jul 12, 2013 at 4:37 PM
    #3
    Tacoyota

    Tacoyota senile member

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    try to find the sizes approx. by height. So lets say one is 31", and you went to 33".

    So 33"/31"=1.0645 these are always approximate. If you had a good distance each rolled per turn it could be more accurate.
    The 1.06 is going to be what you need to multiply your tripmeter by to get actual miles closer.

    Better yet, just keep close track with a GPS, fill up at the same pump and same setting.

    Use the tire size calculator uner the "mods and tutorial" near the top of the screen too.
     
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  4. Jul 12, 2013 at 4:45 PM
    #4
    JDMcQ

    JDMcQ Well-Known Member

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    Nothing to do with it. All e needs is his distanced traveled and volume of fuel used.
     
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  5. Jul 12, 2013 at 6:01 PM
    #5
    Erik65

    Erik65 Dog slave

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    Did you recalibrate the speedo/odo when you upsized (e.g.--Hypertech calibrator)? If not you will be off a little on miles to calculate MPG...
     
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  6. Jul 12, 2013 at 7:34 PM
    #6
    Goosedog

    Goosedog [OP] Well-Known Member

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    Thanks, that would make sense. How does one go about that?
     
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  7. Jul 12, 2013 at 11:18 PM
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    acuracing

    acuracing New Member

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    Interested about this recalibration as well.
     
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  8. Jul 15, 2013 at 6:10 AM
    #8
    Gearheadesw

    Gearheadesw must modify

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    If a tire is 10% bigger in diameter then the stock tire, it's 10% bigger in almost all respects, rolling distance, revolutions per mile etc. Add 10% to the miles traveled, and divide by the gallons consumed. Close enough for me.
     
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  9. Jul 15, 2013 at 3:20 PM
    #9
    Erik65

    Erik65 Dog slave

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  10. Jul 15, 2013 at 7:04 PM
    #10
    Boilerman

    Boilerman Well-Known Member

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    Really? Then how come when I went from P245/75/16 Dunflops to LT265/75/16 BFG All Terrains, I lost two mpgs? Doing this made my speedo correct to my GPS? Going to this tire was a 16 lb. per tire increase, so yes weight is a factor. No different if you replaced your front bumper with a heavy duty full replacement bumper with grill guard. More weight = less mpgs.
     
  11. Jul 15, 2013 at 7:09 PM
    #11
    KenLyns

    KenLyns 8.75" Third Member

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    ^There are two ways of interpreting OP's question.

    1. Predicting MPG loss with larger tires.
    2. Already installed larger tires, need to correct milage with larger rolling dia.

    For #2, you need the "rev per mile" info from the tire mfr's web site. For example, the Dunlop AT20 P245/75R-16 that's OEM on Tacoma SR5 is 686 revs/mile.

    The BFG All-Terrain in LT265/75R-16 is 654 revs/mile.

    So if you calculated 17.0 mpg from the gas pump reading and odometer reading, your actual mpg is 17.0 x 686 / 654 = 17.8 mpg

    It's better to use mfr's data, as two tire models of the same P-metric size can vary by 6%.
     
    Last edited: Jul 15, 2013
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  12. Jul 15, 2013 at 7:13 PM
    #12
    Boilerman

    Boilerman Well-Known Member

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    Not referring to Op's question, referring that extra weight does not affect mpgs. Go ahead and throw an extra hundred lbs in the bed of your truck and see if your mileage doesn't go down.
     
  13. Jul 15, 2013 at 7:30 PM
    #13
    worthywads

    worthywads Well-Known Member

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    No one said extra weight didn't drop mpg.
     
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  14. Jul 15, 2013 at 8:17 PM
    #14
    Boilerman

    Boilerman Well-Known Member

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    Go back to jdmcq's response. He is referring to weight.
     
  15. Jul 15, 2013 at 8:34 PM
    #15
    worthywads

    worthywads Well-Known Member

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    But the context of jdmcq's response was about the op's question, which wasn't about weight. jdmcq did not say weight doesn't matter.

    Your speedometer might now be correct but if your odometer was accurate before you put on bigger tires it isn't now.
     
  16. Jul 15, 2013 at 8:42 PM
    #16
    Boilerman

    Boilerman Well-Known Member

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    So if weight doesn't matter, then according to jdmcq size of tire doesn't matter either???
     
    Last edited: Jul 15, 2013
  17. Jul 15, 2013 at 9:12 PM
    #17
    worthywads

    worthywads Well-Known Member

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    Sure it does, he referred to distance traveled which changes, that's what op wants to know how to calculate.
     
  18. Jul 15, 2013 at 9:29 PM
    #18
    Boilerman

    Boilerman Well-Known Member

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    Modern cars are designed to work as a complete package. A car's fuel economy is a factor of engine efficiency, transmission and differential gear ratios, aerodynamics, wheel size and weight and tire size and construction. Car manufacturers use all of these factor to maximize the efficiency. Changing one of the factors---tires, for instance---without taking the others into consideration will usually reduce the fuel efficiency of the car.

    Changing the tires on a car so they're taller or shorter will provide a false change in perceived fuel economy. The speedometer of a car is calibrated to the revolutions per mile of the original tires, and changing tire size will result in an inaccurate odometer reading. Shorter tires will take more revolutions per mile, resulting in a higher calculated gas mileage using the uncorrected odometer reading. In fact, shorter tires will reduce fuel efficiency because the engine must run at a higher rpm to achieve the same speed and travel the same distance.

    If shorter tires reduce fuel mileage, do taller tires increase fuel economy? If you could get a taller tire without changing any other factors, the theoretical answer is yes. A taller tire will increase the weight and rolling resistance. However, the taller tire is also not designed for the gearing of the engine and drive train. If the car is a newer model, the computer will not know there is a taller tire and may cause the vehicle to run poorly and use more gas. Changing to taller tires will result in fewer tire revolutions per mile, but the offsetting factors will probably negate any theoretical fuel economy .
     
  19. Mar 26, 2016 at 7:26 AM
    #19
    Kyitty

    Kyitty Mr. Beard

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    I'm sorry to resurrect this thread but........OMG.

    :goingcrazy:

    Some of the arguing over tire weight is....OMG.

    Yes, tire weight impacts actual fuel consumption. But the OP's question was mathematical for crap sake! Tire weight does NOT factor into the formula used to calculate how many miles you drove on a given quantity of gasoline.

    Last I checked MPG calculating was: Miles Driven / Gallons Used = MPG
    It's pretty damn simple.

    The difference in tire SIZE throws off your odometer reading. So, if you know the change factor in revolutions per mile from factory tires to aftermarket tires you compensate like such:

    Dunlop Grandtrek 245/75/16: 686 revs per mile
    Cooper ST Maxx 255/85/16: 629 revs per mile

    686/629 = 1.09 (This shows a 9% variance in revs per mile).
    Your vehicle was programmed to measure 686 revolutions at the wheel to measure a mile driven. So the new formula if reading change in odometer (or trip meter) is:

    (Miles Driven * 1.09) / Gallons used = MPG

    Let's say the odo read 100 miles were driven and you used 5 gallons of gas.
    100/5= 20 MPG

    To compensate for an inaccurate odo due to my previously mentioned tire sizes:
    (100*1.09)/20 = 21.8 MPG

    Because the larger tire revolves fewer times in a mile your odometer reading will be LOWER than reality. So because there was a 9% variance we are adding in that 9% shortage in miles driven to determine how many miles were actually driven.

    No where does the weight of your tires factor into this simple formula to determine how many miles per gallon you attained.

    Again - Tire weight does impact the amount of energy needed to cause rotation. So weight DOES impact how many MPG you WILL get. But it has nothing to do with calculating the MPGs you DID get.
     
    Last edited: Mar 26, 2016
  20. Mar 26, 2016 at 9:19 AM
    #20
    worthywads

    worthywads Well-Known Member

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    I gave up after that last post, good luck getting some to admit the original question was strictly about odometer or distance accuracy and leave it at that.
     

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