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    Tire ratio effecting speedo reading?

    Currenty have a 100/90 on the front of the Cooley and I passed one of those radar booth things at a construction site and the reading said I was doing 30..but the speedo said 35.

    So this got me to thinking that would this be to the tires circumference throwing the speedo off?? If so, what would the correct tire be to make it all read relatively close to right????
    MY BIKES..1977 GS 750 B, 1978 GS 1000 C (X2)
    1978 GS 1000 E, 1979 GS 1000 S, 1973 Yamaha TX 750, 1977 Kawasaki KZ 650B1, 1975 Honda GL1000 Goldwing, 1983 CB 650SC Nighthawk, 1972 Honda CB 350K4, 74 Honda CB550

    NEVER SNEAK UP ON A SLEEPING DOG..NOT EVEN YOUR OWN.


    I would rather trust my bike to a "QUACK" that KNOWS how to fix it rather than a book worm that THINKS HE KNOWS how to fix it.

    #2
    If my thinking is right, a smaller tire would spin the hub faster, thus making the needle read above actual speed..right? Maybe the speedo is just that far off and I gotta adjust.
    MY BIKES..1977 GS 750 B, 1978 GS 1000 C (X2)
    1978 GS 1000 E, 1979 GS 1000 S, 1973 Yamaha TX 750, 1977 Kawasaki KZ 650B1, 1975 Honda GL1000 Goldwing, 1983 CB 650SC Nighthawk, 1972 Honda CB 350K4, 74 Honda CB550

    NEVER SNEAK UP ON A SLEEPING DOG..NOT EVEN YOUR OWN.


    I would rather trust my bike to a "QUACK" that KNOWS how to fix it rather than a book worm that THINKS HE KNOWS how to fix it.

    Comment


      #3
      Glad to hear you are back in the saddle ! Maybe you just need "go fast air" in those tires.
      82 gs1100e FAUX Skunk
      80 gs1000s

      Comment


        #4
        Chuck, what bike are you asking about and does your phone have GPS or do you have a navigation device of some sort?

        Compare the GPS reading to the indicated speed on the speedometer, over a range of vehicle speeds, and let me know what they are.
        De-stinking Penelope http://thegsresources.com/_forum/sho...d.php?t=179245

        http://www.thegsresources.com/_forum...35#post1625535

        Comment


          #5
          My experience has been that those portable radar speed signs are almost never correct. I think they're supposed to be calibrated from time to time or something. I've seen them 0-5 mph off in either direction, doesn't seem to be any consistency.
          Charles
          --
          1979 Suzuki GS850G

          Read BassCliff's GSR Greeting and Mega-Welcome!

          Comment


            #6
            No got a GPS available Dale. If i recall correctly the stock tire is a 100/90 and the back is a 120/90??? The speed booth thing got my eye and now my curiosity going.
            MY BIKES..1977 GS 750 B, 1978 GS 1000 C (X2)
            1978 GS 1000 E, 1979 GS 1000 S, 1973 Yamaha TX 750, 1977 Kawasaki KZ 650B1, 1975 Honda GL1000 Goldwing, 1983 CB 650SC Nighthawk, 1972 Honda CB 350K4, 74 Honda CB550

            NEVER SNEAK UP ON A SLEEPING DOG..NOT EVEN YOUR OWN.


            I would rather trust my bike to a "QUACK" that KNOWS how to fix it rather than a book worm that THINKS HE KNOWS how to fix it.

            Comment


              #7
              Not sure about your 1000, but the smaller bikes came with 90/90-19s on the front and were rather optimistic on the speedo readings. I have put 100/90s on all our bikes and have gotten the readings quite a bit closer to actual speeds. Now, an indicated 60 shows up as 57 to 59 on my GPS, depending on the bike. That little bit of error I can live with.

              In order to slow down your speedo, you would have to fit a 110/90, not sure if that would fit.

              .
              sigpic
              mine: 2000 Honda GoldWing GL1500SE and 1980 GS850G'K' "Junior"
              hers: 1982 GS850GL - "Angel" and 1969 Suzuki T250 Scrambler
              #1 son: 1986 Yamaha Venture Royale 1300 and 1982 GS650GL "Rat Bagger"
              #2 son: 1980 GS1000G
              Family Portrait
              Siblings and Spouses
              Mom's first ride
              Want a copy of my valve adjust spreadsheet for your 2-valve per cylinder engine? Send me an e-mail request (not a PM)
              (Click on my username in the upper-left corner for e-mail info.)

              Comment


                #8
                But on say a 100/90 -19 tire isn't the 100 the width? Why would increasing the width (say from a 90/90-19 to a 100/90-19) affect the speedo?

                Comment


                  #9
                  70% is sidewall and 30% is tread width. So the 100 would be slightly taller/larger diameter than a 90 tire.

                  Comment


                    #10
                    the second number "90" is the aspect ratio, or percentage of the width. so 90% of 100 would be greater than 90% of 90, which is the height, or overall circumference of the tyre.
                    1978 GS1085.

                    Just remember, an opinion without 3.14 is just an onion!

                    Comment


                      #11
                      Originally posted by greasyjonny View Post
                      But on say a 100/90 -19 tire isn't the 100 the width? Why would increasing the width (say from a 90/90-19 to a 100/90-19) affect the speedo?
                      Because a wider tire is also a taller tire. A taller tire will roll a bit farther on each revolution, which will slow down the speedo.


                      Originally posted by GSX1000E View Post
                      70% is sidewall and 30% is tread width. So the 100 would be slightly taller/larger diameter than a 90 tire.
                      I agree with the 100 being larger than the 90, but where the heck did that 70/30 thing come from?

                      .
                      sigpic
                      mine: 2000 Honda GoldWing GL1500SE and 1980 GS850G'K' "Junior"
                      hers: 1982 GS850GL - "Angel" and 1969 Suzuki T250 Scrambler
                      #1 son: 1986 Yamaha Venture Royale 1300 and 1982 GS650GL "Rat Bagger"
                      #2 son: 1980 GS1000G
                      Family Portrait
                      Siblings and Spouses
                      Mom's first ride
                      Want a copy of my valve adjust spreadsheet for your 2-valve per cylinder engine? Send me an e-mail request (not a PM)
                      (Click on my username in the upper-left corner for e-mail info.)

                      Comment


                        #12
                        Ok I got it. Makes sense, I knew increasing the diameter would affect it but I was ignorant to he fact that increasing width also increased the height. Maybe i should have bought those 100-90/19 Avons instead of the 90/90s.

                        Comment


                          #13
                          Originally posted by Steve View Post
                          Because a wider tire is also a taller tire. A taller tire will roll a bit farther on each revolution, which will slow down the speedo.

                          I agree with the 100 being larger than the 90, but where the heck did that 70/30 thing come from?
                          .
                          Ask themess (sp?) he worked at making tires.

                          Comment


                            #14
                            Hey Chuck .
                            The speedo on my 'S reads a bit "fast".
                            I got Deb to run at 100kph in our car (a reasonably late model Toyota) on a straight bit of road and checked my speedo against that . It was reading 110kph .
                            I use that and the tacho (4,500 rpm @ 100kph in top gear) to get my speed right .
                            Surprisingly , my odometer reads fairly accurately .
                            It didn't freak me out enough to pull my speedo apart to get it perzactly correct , I just adjusted me .

                            Don't spin out mate , it's not a perfect world (and working out tyre sizes and $hit will do your head in).

                            Cheers , Simon .
                            http://i258.photobucket.com/albums/h...esMapSimon.jpg

                            '79 GS1000S my daily ride in Aus

                            '82 (x2) GS650ET in the shed

                            Comment


                              #15
                              Im not freaked about it Simon. I just want to know how much off it is so I can also adjust myself to what it reads. Dang ticket is not what I need. If its 5 over then i can adjust from that point.
                              MY BIKES..1977 GS 750 B, 1978 GS 1000 C (X2)
                              1978 GS 1000 E, 1979 GS 1000 S, 1973 Yamaha TX 750, 1977 Kawasaki KZ 650B1, 1975 Honda GL1000 Goldwing, 1983 CB 650SC Nighthawk, 1972 Honda CB 350K4, 74 Honda CB550

                              NEVER SNEAK UP ON A SLEEPING DOG..NOT EVEN YOUR OWN.


                              I would rather trust my bike to a "QUACK" that KNOWS how to fix it rather than a book worm that THINKS HE KNOWS how to fix it.

                              Comment

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