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Speedometer is off - is it the tire size?

Discussion in 'XJ Technical Chat' started by Niels Keiser Jacobsen, Apr 24, 2017.

  1. Niels Keiser Jacobsen

    Niels Keiser Jacobsen New Member

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    Hi all.

    I am new to motorcycles, and I fell in love with a lovely xj650 (UK version), and have now been riding for about a week.
    I'm from Denmark, so I'm a bit weak in the inch, miles etc. system, so please bare with me.

    The previous owner told me, that the speedometer is showing 10mk/t (6,21mph) too low, so when I'm riding 50kmt (31mph), it only shows 40km/t (25mph).

    The original front tire size is 3.25H19-4pr, which I understand is about 82, in tire with.

    The new front tire is a 100/90-19, which people seem to say is the correct modern tire size for the bike.
    Doesn't that affect the speedo?

    I have used a webpage to look up speedo conversions, where I filled in the specs for the original 3.25 tire, assuming that the aspect ratio is 90 (but is it?), then I got the data as shown in the picture.
    Skærmbillede 2017-04-24 kl. 09.32.11.png

    As the picture indicates, the speedometer readings are increasingly off, the faster the wheel turns (which makes sense).
    So my question is: Does everybody sporting a new 100/90-19 tire have the same readings, or does my problem lie somewhere else?

    Many thanks
     
  2. Niels Keiser Jacobsen

    Niels Keiser Jacobsen New Member

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    woops.. I noticed that I needed the diameter of the tire, and the width, and not width and aspect ratio.
    The new data says:

    Skærmbillede 2017-04-24 kl. 09.52.45.png

    So this indicates that it doesn't really make that much of a difference changing to a 100/90-19 tire?
    So my speedometer problem is supposedly originating somewhere else?
     
  3. TheCrazyGnat

    TheCrazyGnat Well-Known Member

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    Is your speedometer original to the bike? Generally speaking, I have heard that the stock speedometer/tire combination is "optimistic" (the speedometer actually reads higher than you are actually going.) That is my experience with the stock configuration.
     
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  4. Niels Keiser Jacobsen

    Niels Keiser Jacobsen New Member

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    I am quite sure that the speedo is original.
    Yes I am aware that the speedo's normally reads a little higher, but if it reads 10km/t higher at such low speeds, then I am a bit unsure about how it reads at higher speeds. But I have to go out with a gps to measure it correctly.

    It just confuses me, that it can be that much off, if the tires are supposed to match. :)
     
  5. TheCrazyGnat

    TheCrazyGnat Well-Known Member

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    I think the smaller diameter tire should actually make the speedometer read higher than it normally does. I would think this might indicate something is wrong with the speedometer or the magnet maybe? If wait on one of the more knowledgeable members to weigh in though.

    I might check out this thread.
     
    Last edited: Apr 24, 2017
  6. XJ550H

    XJ550H Well-Known Member Premium Member

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    have no problem with the speedo accuracy. I test it when ever I see a radar box and it seams to be very accurate at all speeds
     
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  7. Niels Keiser Jacobsen

    Niels Keiser Jacobsen New Member

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    ah, I didn't think about locating a radar box, and test it there. I should try that!
    It's just a bit unnerving to know that the previous owner said "btw. it reads about 10km/t lower than the actual speed at around 50km/t" :p
     
  8. Greggy

    Greggy Member

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    You can get a app for phone's that's a GPS speedometer you can try that, that's what I use on my bike
     
  9. Niels Keiser Jacobsen

    Niels Keiser Jacobsen New Member

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    So your Speedo is off as well?
     
  10. cgutz

    cgutz Well-Known Member

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    My XJ550 speedo is spot on when I see a radar box.
     
  11. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    The greatest issue with a spedometer that age is the magnet being slowly de-magentized (it's just the nature of that type of magnet). This will result in less drag on the speedometer needle cup, and an innacurate speedometer. Some demagnetize at a faster rate than others, and not all speedometers were terribly accurate to begin with (there is variation even from the same manufacturing lot). There is a DYI method for re-calibrating them, but it's fiddly and time consuming. There are several shops that offer the service (though you'll have to do a search as all my contacts are in the U.S.). Or you can take a shortcut and make labels to stick on the glass and "recalibrate" the speedometer.
     
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  12. Greggy

    Greggy Member

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    When I bought my bike there was no speedometer, tach or horn so I got a phone holder that goes on the handle bars and a speedometer app for my phone, it works good and a horn, I can do without the tachometer
     
  13. waldreps

    waldreps Active Member

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    My speedometer shows about 6 mph slower than I'm actually going at around 65 mph. It's dead on up until about 35 mph and then it starts to lag behind more and more as I go faster.
     
  14. vashtsdaytona

    vashtsdaytona Active Member

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    for me I have a mounted GPS and compare my speed often, for simplicity sake, mine reads 10miles SLOW. so an indicated 60 is 70
     
  15. Niels Keiser Jacobsen

    Niels Keiser Jacobsen New Member

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    Update!
    Thanks for all the replies!

    I have been riding some longer trips, including a lot of highway, and it seems to me, that my speedometer reads 10km/h constantly, so that I just have to add 10km/h to the speedometer-reading.

    So it seems to me, that it's not worth the hassle, to fix it :)
    Unless I start to get speeding-tickets - because that can of course only be because of the speedo's misreading ;) (*joke)
     
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  16. Paul Howells

    Paul Howells Active Member

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    If you have time to look at your speedo then you are not going fast enough.
     
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  17. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Someone from the county will be by shortly to let you know if you're going too fast ;)
     
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