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  • New Tires and Speedo Question

    I just got done doing my lift and bigger tires...but my speedo gear must be on back order or something.

    Previous Setup - 31" BFG's, 3.73 gears and a 34 tooth speedo gear

    Now - 33" ProComps, 3.73 gears and the same 34 tooth speedo gear

    My question...where am I running for speed? If my speedo says 55...am I running below that or above that and by approximatly how much?

  • #2
    You're running above that, so slow down before you get a ticket!
    You can do a calc on tire circumferences to find the difference between your current distance traveled per rotation vs. old distance traveled.
    That's too much math for my brain this morning.

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    • #3
      Try this site several different calculators to use http://www.ringpinion.com/Calculators.aspx

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      • #4
        I made my own spreadsheet for this stuff. Assuming the tires are actually 31" and 33" tall respectively....you are 6% off. Which means when your speedo is showing 52.7...you are really running 55 mph. And when it shows you 61...you are really running 65 mph.
        Last edited by Clifford; 04-23-2008, 10:50 AM.

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        • #5
          Cool, that will get me by until my gear comes.

          Thanks guys.

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          • #6
            Originally posted by 1 MAD TJ View Post
            Cool, that will get me by until my gear comes.

            Thanks guys.
            Out of curiosity which teeth speedo gear did you get? I have the same set up and used an online calculator which gave me 30 teeth, but all the charts I've seen states 31 or 32

            I went with a 30 but have a bad feeling about it.

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            • #7
              I went with a 30. Once I get it in, I will get the gps out and see what it says.

              Previously, my jeep had 31's on it and I was 1 tooth off of what the charts says and was running pretty close. So I am hoping this will be the case too. If not, I will order up a 31 and give it a shot.
              Last edited by 1 MAD TJ; 04-23-2008, 03:25 PM.

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              • #8
                To add to your entertainment, as I discovered with my new Libby, the speedo is 3% off (so I'm going 3% slower than it reads)... after the dealership doublechecked every Libby on their lot, and redid mine a couple of times, they called Jeep tech, and the results:??:
                DOT allows up to a 5% variance before it's a "problem"... so as long as you get "close enough" you're good.

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                • #9
                  Originally posted by 1 MAD TJ View Post
                  I went with a 30. Once I get it in, I will get the gps out and see what it says.

                  Previously, my jeep had 31's on it and I was 1 tooth off of what the charts says and was running pretty close. So I am hoping this will be the case too. If not, I will order up a 31 and give it a shot.
                  I already have mine installed but I won't have a GPS until Friday. I'll let you know how it goes if you want, so you can change your order if necessary.

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                  • #10
                    jayshomp...what are you doing with a picture of my jeep in your avatar?

                    https://home.mchsi.com/~sdohman/JeepBuild/tjafter6.jpg

                    But do let me know what you come up with, as 4wd has yet to even process that part of the order. I recieved the rest of my order, but the gear must be on back order.

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                    • #11
                      Haha, nice. Mine's only a 2.5L though. Trade ya And use some Krylon Fusion on those flares, works great! :P

                      Yes they're definitely on BO, and at more places besides here. Took me awhile to find a place that had them.

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                      • #12
                        some fusion is already in the plans. I was thinking of going with some flat black. I don't want them shiny!!

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                        • #13
                          Well, I had the GPS out tonight to check it before I put in a new gear and it is running about 3 mph over at 60. I will see what it is later when I get the new gear.

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                          • #14
                            Originally posted by 1 MAD TJ View Post
                            Well, I had the GPS out tonight to check it before I put in a new gear and it is running about 3 mph over at 60. I will see what it is later when I get the new gear.
                            I got my GPS a day early so here ya go.

                            At 50mph, my GPS reads 47.5mph

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                            • #15
                              Originally posted by 1 MAD TJ View Post
                              Well, I had the GPS out tonight to check it before I put in a new gear and it is running about 3 mph over at 60. I will see what it is later when I get the new gear.
                              I wouldn't even bother with a new gear for 3mph, IMO.

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