Milage variance with bigger tires?

Discussion in 'Wheels/Tires' started by cvsisinthehouse, Jan 29, 2018.

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  1. Jan 29, 2018 #1

    cvsisinthehouse

    cvsisinthehouse

    cvsisinthehouse

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    I used to get to about 114 miles before the fuel light came on with the stock tires, now I have a 170 on the rear, 150 on the front and it comes on around 90 miles? Does this sound right? Can someone explain it to me?
     
  2. Jan 29, 2018 #2

    Fire-medic

    Fire-medic

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    Are you using a stock front rim? If so, you are way oversized. Your rim can take a 120 but anything over that, and the tire profile on the rim is very distorted. The tire plys are designed for a certain amount of flex, and 'spread,' but running something so oversize, if it's a stock rim could put you at risk of rolling a tire right-off its bead, and overheating the tire. The #1 problem is that you are causing your tire contact patch to be smaller than a stock tire though you think it's bigger, because it's a larger tire. It's a similar effect to over-inflating your tire. Avon recommends a minimum 3 inch rim to fit a 150 tire, stock is a 2.5" X 18" front.

     
  3. Jan 30, 2018 #3

    cvsisinthehouse

    cvsisinthehouse

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    Its NOT the stock front wheel. Its a Madmax one, had a 130 on it when I got it.
     
  4. Jan 30, 2018 #4

    Fire-medic

    Fire-medic

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    What's the width between flanges? I usually get about 93 miles out of a tank, and I have a 17" rear.
     
  5. Jan 30, 2018 #5

    MaxMidnight

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    If the rolling radius of the tyre is greater than OE then for each turn of the wheel you will travel further for each revolution.
    The odometer is counting the number of revolution of the wheel and converts this into mileage.
    Thus if for the OE tyre it took (say) 10000 revolutions to cover your 114 miles it may only take (say) 9000 with a larger rolling radius.
    As far as the speedo is concerned you have therefore covered less miles and thus thus the light comes on at ninety miles.
     
  6. Jan 30, 2018 #6

    Bill Seward

    Bill Seward

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    You'll also notice the tachometer will read more rpm at indicated speeds with the larger tire in front..

    Size in this case means overall diameter, not tread width.
     
  7. Jan 31, 2018 #7

    MaxMidnight

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    Bill, I'd be inclined to reverse that statement as the tacho reading for any actual road speed would be the same but the point you make is valid

    i.e. The indicated road speed will be higher at any r.p.m.
     
  8. Jan 31, 2018 #8

    Bill Seward

    Bill Seward

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    Larger diameter front tires "slow down" the indicated speed shown on the speedometer. Less wheel revolutions per distance travelled. The actual road speed rises compared to the indicated speed. So the tach reading world rise at any indicated road speed due to the slowing of the front wheel revolutions per mile.

    Assume 60 mph actual road speed with a stock front wheel diameter = 4000 rpm. Now change to a tire with 10% more circumference. To get that speedo to read 60, you'd need a actual road speed of 66 mph. 66 actual mph would need an engine speed of 4400 rpm. You'd be seeing an inaccurate road speed of 60 while actually moving 66 mph. Your tachometer would read engine revs that are driving the rear wheel which didn't change circumference. So engine speed would appear to go up due to the inaccuracy of the speedo caused by the larger circumference of the front wheel. This would also affect fuel mileage as indicated by the odometer, as the indicated miles are longer than real miles. You'd observe a decrease in indicated miles to reserve. Remember the indicated miles are longer than real miles due to the slower revolution of the larger front wheel..
     
  9. Jan 31, 2018 #9

    cvsisinthehouse

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    Some heavy reading here! Thank you for the explanations. So a taller front tire throws off the speedometer and odometer, about 20% in my case. What does a larger rear tire do to the speedo and odo? Nothing right? as they are solely based on front wheel turns. I guess a taller rear tire only means your engine doesnt work as hard as before. Right? I mean, if you go a little farther with every revolution, then it doesnt need to turn as much to get where you are going? Does that sound right? The tach is the tach, its always actual- right?

    I better watch those speed cameras that let you go 11 mph over, but not 12, that are all over my county/commute, since my speed is more than my gauge says.
     
  10. Jan 31, 2018 #10

    93max

    93max

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    Larger diameter in the rear will also take away some of the bottom end too.
     
  11. Jan 31, 2018 #11

    Bill Seward

    Bill Seward

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    Gen 1 has a front wheel driven mechanical speedometer. Rear wheel diameter has no effect. Changing wheel or tire size to a different diameter on the rear will definitely affect overall gearing. Bigger diameter will gear bike up - RPM at specific road speed will be less with a bigger diameter wheel. Again, this is the Diameter of the wheel/tire combo - NOT the width of the tire.
     
  12. Feb 6, 2018 #12

    carrizog60

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    changing to 180-55-17 at rear and 120-70-17 at front will keep it almost equal or will it change enough to notice?
     
  13. Feb 6, 2018 #13

    Bill Seward

    Bill Seward

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    That's a good question. You'd have to compare the circumference of the new wheels compared to the old ones. Back for overall gearing, front for speedometer reading..

    Keeping in mind that motorcycle speedometers are notorious liars...
     
  14. Feb 6, 2018 #14

    VMAXXIMUM

    VMAXXIMUM

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    I can vouch for that. Mine is always telling me I'm going faster than I really am.
     

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