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    speedo

    my speedometer seems to be 5mph slow at 25 and 10 mph slow at 50.. i'll be going 50 and my gps will show 60. I recently did an engine/trans swap but I also swapped the speedo gears since the new car has a different rear end ratio.

    where do i go from here?
    Give a man a fish and he will be fed for a day. Teach a man to fish and he will promptly forget that he once did not know, and proceed to call anyone who asks, a n00b and flame them on the boards for being stupid.

    #2
    sounds like the speedo gear you used is just a smidge off. Might need a different gear.

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      #3
      How did you verify your speed? GPS? Roadside radar? I don't trust mile markers any more. They pull them out during construction, and put them back, sorta in the right place.

      You'll need to change the driven gear. One or two less teeth (typically you see a 3-5 mph change per tooth. More teeth, it'll spin slower, less teeth it'll spin faster). Did you count the number of teeth on the gear you put in?

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        #4
        he said he checked it via gps

        Originally posted by cld783 View Post
        i'll be going 50 and my gps will show 60.
        Originally posted by gadget73
        There is nothing more permanent than a temporary fix.
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          #5
          Why is this in the engine section?


          Speedo errors can be corrected by swapping gears. Your Ford dealer can get them for you cheaper than online, just find the percentage your speed is off (and in which direction), pull your current speedo gear and count the teeth, and have them get you the one that corrects the situation. If the one you need is something common like a 16-tooth, some of us can send you one for the price of shipping, but getting it from the dealer may still be even cheaper.
          2012 Mazda5 Touring | Finally working on the LTD again!

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            #6
            Originally posted by 1987cp View Post
            Why is this in the engine section?

            No matter how easy we make it, there are still those who don't get what the sections are for.
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              #7
              Originally posted by Jonathan View Post
              No matter how easy we make it, there are still those who don't get what the sections are for.
              I forgot about the subsections..


              I verified the car at 5 mph - 10 mph slow with a gps unit. I even took the speedo gear out of the original tranny with the same result.

              both speedos from different cars had a 17t one gear said 17t 2d, the other one said 17t 3d.. one car was a 3.08 open and this one is a 2.73..
              Give a man a fish and he will be fed for a day. Teach a man to fish and he will promptly forget that he once did not know, and proceed to call anyone who asks, a n00b and flame them on the boards for being stupid.

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                #8
                MPH error without a reference point doesn't help. Percentage slow or fast does help. Or even something like "observed 75 was actually 120".



                I've also noticed funny things with respect to a car liking a different driven gear that you'd expect. Logically, the same RAR and the same output shaft will want the same driven gear. My white wagon came to me with 3.27s and an 18-tooth gear, and I never noticed any problems. With 28" tires, it wanted a 16-tooth. The only other thing that'd changed was swapping speedos, so maybe they speedos were just responding differently? Who knows!

                Logically, going from 27" tires to 28" would only make the speedo read about 3% low, which shouldn't have really caused problems (observed 70 would actually be 72.5; observed 35 would actually be 36). It ended up being off enough that I not only kept consistently ahead of traffic in an area where EVERYONE speeds, but eventually attracted police attention while thinking I was observing the speed limit. The 16-tooth gear that was the cure should have been needed to correct a 12.5% error, not a 3% error. Go figure!

                On the other hand, my 3.27:1 sedan came to me with a 16 tooth gear and the speedo reading high. Randomly putting in the 18-tooth gear from the wagon made it read spot-on with either 27" or 27.5" tires. So this one behaved correctly, and the other one did not!
                2012 Mazda5 Touring | Finally working on the LTD again!

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                  #9
                  if the tripminder displays the same exact speed as my speedometer, wouldnt that tell me the speedometer and cable is good? I just don't get why using the same speedo gear that ford installed for the rear end would make that much of an error.

                  maybe it was like this before i even swapped the engine/tranny and just never noticed?? confused
                  Give a man a fish and he will be fed for a day. Teach a man to fish and he will promptly forget that he once did not know, and proceed to call anyone who asks, a n00b and flame them on the boards for being stupid.

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                    #10
                    Originally posted by cld783 View Post
                    if the tripminder displays the same exact speed as my speedometer, wouldnt that tell me the speedometer and cable is good? I just don't get why using the same speedo gear that ford installed for the rear end would make that much of an error.

                    maybe it was like this before i even swapped the engine/tranny and just never noticed?? confused
                    im sure it was, my '88 came from the factory with a speedo gear that was too big.
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                      #11
                      First... make sure you are using the correct the driven gear for your RAR and tire size.

                      Read here: http://www.grandmarq.net/vb/showthre...t=speedo+gears

                      Second... take the speedo head out and find a shop that can calibrate it. Speedo needle in my car was off and not pointed at ZERO when car was stopped. there is also a "hair" spring on the back the needle that can be adjusted, but you need some fairly accurate equipment to do this.
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