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Tire Size vs Gas Mileage Debate

Discussion in 'Wheels & Tires' started by Rocketball, Aug 29, 2009.

  1. Aug 29, 2009 at 9:05 AM
    #1
    Rocketball

    Rocketball [OP] If The World Didn't Suck, We'd All Fall Off

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    I've been thinking about the debate/concerns that a lot of folks have regarding the loss of gas mikeage when going to a larger tire size, so I sat down and did some math to see if there was a connection..... This model does not account for the actual rolling circumference of each tire. I know that is slightly different than the stated circumference. I have also rounded the numbers.

    Here's my thoughts and the math to back it up. I used my 08 DC Sport as my model and The Tire Rack to get the tire specs:

    My 08 Sport came with Dunlop AT20's, 265/65R17. This tire has a diameter of 30.6", so the tire circumference is 96.13".

    I upgraded to Bridgstone AT's, 265/70R17. This tire has a diameter of 31.9", so the tire circumference is 100.22".

    With the larger size, you are covering a slightly longer distance with each rotation of the tire, 4.09" to be exact in this model. So Unless you can recalibrate your speedo, or use a scanguage, using the odometer to calculate your fuel mileage will be incorrect.

    Without boring everyone with the speadsheet I did to calculate the tire rotations per mile, let's just say in this model, the larger Bridgestone tire will cover an additional 12.75 miles over the Dunlop tire in 300 miles.

    That's in additional 4.2% longer distance than the Dunlop tire.

    I just checked my fuel mileage for the first time since getting my new tires, and in an average tank driving to and from work, it was 18.7 mpg (287 miles / 15.3 gallons). The last time I checked it with the Dunlop tires it was 19.5.

    So I actually traveled 299 miles given the larger tire size. 287 x 104.2% = 299.05 miles.

    Hmmmmmm....... According to my calculations I'm getting exactly the same mpg with my larger tires because I actually traveled 4.2% longer distance. 287 odometer miles x 104.2% = 299.05 miles / 15.3 gallons = 19.54 mpg.

    If there are any flaws in my math here please let me know.

    I know many folks here say there's additional rotational weight associated with larger tires that will hamper mpg, but with this small tire size increase I don't believe it's an issue.
     
  2. Aug 29, 2009 at 11:23 AM
    #2
    Krazie Sj

    Krazie Sj Resident Jackass

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    I can't see adding another 30lbs of tire killing your MPG that much. A couple hundred sure, but the truck can haul 6500lbs. While any parasitic weight would be detrimental to mpg, I see it the equivalent of wearing a watch and carrying my wallet around with me.
     
  3. Aug 29, 2009 at 11:38 AM
    #3
    gvidas

    gvidas Member

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    I agree that it's important to account for speedo inaccuracy when having this conversation. A simpler shortcut might just be to use a GPS.

    I suspect a much larger factor in fuel economy than tire diameter is tread pattern. Rolling resistance is related to the tire surface in ways I don't entirely understand, although you can kind of summarize to the point that smooth tires at higher pressure will roll better than low pressure knobbie tires.

    In a general sense I think the conversation about tire size vs mileage is probably confused by the fact that people who are putting on large tires typically are going to put on an all terrain or offroad tire, which is probably designed to flex and grip to a degree that you really don't need on flat pavement.
     
  4. Aug 30, 2009 at 6:27 AM
    #4
    Rocketball

    Rocketball [OP] If The World Didn't Suck, We'd All Fall Off

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    I'm sure the rolling resistance of a large M&S tire would effect the mpg, but there seems to be a lot of discussion on this board about loosing mpg when only going from a road tire to an AT tire that is slightly larger tire than stock.
     
  5. Aug 30, 2009 at 6:36 AM
    #5
    rick

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    Tire - 265/65R17 >Search Tires Tire 1 - 265/70R17 >Search Tires
    Section Width: 10.43 in 265 mm
    Section Width: 10.43 in 265 mm

    Rim Diameter: 17 in 431.8 mm
    Rim Diameter: 17 in 431.8 mm

    Rim Width Range: 7.5 - 9 in
    Rim Width Range: 7 - 9 in

    Overall Diameter: 30.56 in 776.22 mm
    Overall Diameter: 31.60 in 802.64 mm

    Sidewall Height: 6.78 in 172.21 mm
    Sidewall Height: 7.30 in 185.42 mm

    Radius: 15.28 in 388.11 mm
    Radius: 15.80 in 401.32 mm

    Circumference: 96.00 in 2438.4 mm
    Circumference: 99.27 in 2521.4 mm

    Revs per Mile: 680.6
    Revs per Mile: 658.2

    Actual Speed: 60 mph 100 km/h
    Speedometer1: 58.0 mph 96.7 km/h

    Speedometer Difference: - Speedometer Difference: 3.403% too slow
    Diameter Difference: - Diameter Difference: 3.3%
     
  6. Aug 30, 2009 at 9:04 AM
    #6
    otter

    otter Well-Known Member

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    Adding unsprung weight, such as tires, makes a much bigger difference than it seems some of you are giving it credit for.
     
  7. Sep 2, 2009 at 6:54 PM
    #7
    Rocketball

    Rocketball [OP] If The World Didn't Suck, We'd All Fall Off

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    I agree, but I'm only talking about going from a 265/65 to a 265/70 tire. Not really that much added weight.
     
  8. Apr 15, 2014 at 5:20 PM
    #8
    CowboyTaco

    CowboyTaco Well-Known Member

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    Talk about thread revival!

    This is the closest thread I can find related to what I'm wondering.

    What makes a larger difference in MPG, Diameter or Weight?

    I recently went from a 265/65/17 (Dunlop) to a 265/70/17 All Terrain. The all terrain that I put on is approx. 10lbs heavier than the stock dunlop. I huge oversight on my part, I know. I'm debating on whether to put 265/65 back on, or a lighter 265/70.

    Everyone that I've asked has had no intelligent response. I would assume weight, but it seems like some folks believe diameter would have the larger impact.

    Thoughts?
     
  9. Apr 19, 2014 at 5:19 PM
    #9
    Gearheadesw

    Gearheadesw Well-Known Member

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    some
    Rotational mass. The leverage it takes to spin up a larger diameter tire is greater, in my opinion, then the effect of weight on milage. More weight, will affect the unsprung weight aspect of your suspension action, more mass to dampen, and control with spring rate. The change in gear ratio with a larger tire is decreasing the revs at speed, making it harder for the engine to keep a certain speed or accelerate.
     
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