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Well, I'll be darned!

Discussion in 'Hangout Lounge' started by tabaka45, Apr 5, 2020.

  1. tabaka45

    tabaka45 Well-Known Member

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    I recently changed the tires on my XJ700. It had Dunlap 404's which were on it when I bought it. Turns out that the front Dunlap was a 110/90/19 rather than a 100/90/19 which is the specified size for the bike. So the po had used a slightly larger front tire. I didn't think much about it, but when I rode the bike I immediately noticed than I was running about 1 to 1 1 /2 mph for a given rpm. Not only that but my mpg went up slightly, maybe 1 mpg. I looked up a tire size comparison (https://www.ajdesigner.com/fl_tire/tire.php) and found that the circumference of the 110 tire was 2.2 inches bigger, which of course effected the speedometer since it has to go an additional 2.2 inches each revolution. Sure enough, 60 mph with the 110 tire represents 61.6 mph with the 100 tire, and consequently a better mpg result. By the way, I replaced the Dunlops with Shinkos and so far I really like them. But it remains to be seen how they wear compared to the Dunlops.
     
  2. JetmechMarty

    JetmechMarty Active Member

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    Those tire sizes are only loosely accurate. Some 110 tires fit. Some will fit, but you can't fit a fart between the tire and the fender. Others don't fit. I measured several mounted and inflated 100/90-19 tires in my garage a few years ago. In case you did not know, the first number is the width of the tire in millimeters, in this case, 100 mm. I got measurements from under 95 mm to over 105 mm, all from the same size tire. I like the ones that run smaller because they turn in faster. It's not a huge difference, but it is a difference. Now that I've found what I like, I stick with it. Yes, the speedometer is about as accurate.
     
  3. XJ550H

    XJ550H Well-Known Member Premium Member

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    if the tire makes the odometer inaccruate the math will be off on your MPG

    so the taller tire would make the odometer off making the odometer 2.2 inches short of distance traveled with each reveloution.

    tire size differances are one of the reasons MC odometers are considered inaccurate
     
    JetmechMarty likes this.
  4. tabaka45

    tabaka45 Well-Known Member

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    That’s right. The larger tire makes the speed and miles lower because the tire has to cover more distance to complete one full revolution and the speedometer works off of the front wheel revolutions. The link I attached illustrates the difference that 2.2 inches makes.
     
  5. Simmy

    Simmy Well-Known Member

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    I believe the conspiracy theorists who claim the manufacturers deliberately give you optimistic readouts.
    This way you'll ride slower, believed to be safer and therefore not as likely to sue them from injuring yourself on their product.
    Hmm.
     
  6. Polock

    Polock Well-Known Member

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    my suzuki gets the mph from the counter shaft, so you can make the speedo read anything you want, except right.
    it's 6 mph fast at 55 with stock gearing. of course i could buy a gizmo for 100$ to make it right.... if i cared
     
    hogfiddles likes this.
  7. tabaka45

    tabaka45 Well-Known Member

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    I have only checked mine with a GPS once and it was off one or two mph. Haven’t checked it with the new tire, but I was mostly interested in the difference with the slightly smaller tire. It was more than I thought.
     
  8. XJ550H

    XJ550H Well-Known Member Premium Member

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    mine matches the speedlimit warning sign at the bottom of the big hill
     
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