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Just bought a 66 tbird and everything is pretty straight, I love the car, but the main impediment to me registering it is the spedomometer, which typically reads about 50-60 when I am going much slower, it also doesn't seem to increase linearly with speed... As if it reads 55 then I step on it, accelerate maybe 15mph and now it says 55.4 ... It is sort of like it is sticking, and bouncing up and down a bit too...

I really want to keep the cool long red line speedo if I can.

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The older mechanical speedometers were cable driven. The cable is more like a very tight coil spring with a small square driver crimped on each end. One end was driven by the transmission the other end drove the speedo head. The speedometer was calibrated to the vehicles tire size and final gear ratio by installing specific speedometer drive gears in the transmission. If at some point the differential or transmission was changed it may just have the wrong drive gear in the transmission. If this is the case it should read either high or low say 5% at all speeds.If it is fluctuating you may have a speedometer cable that is binding.On some models you can unscrew the connector on the transmission side and remove the cable from its' housing and lube it with powdered graphite. When reinstalling it rotate it by hand to match the square drive into the square hole in the speedo head. Unfortunatly some cables can only be removed from the housing at the speedometer end which may require disassembly of the dash to gain acess. The worst possible scenario is that the speedometer itself is failing.They can be repaired but it tends to be a specialized service, read that as expensive. I would try to contact a local T-Bird club and see if you can get advise on a local repair shop if the speedometer head needs repair.

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Yeah that is an idea, there is another spedo in the trunk but I don't know if it was replaced, or the old owner had intended to replace it. –  Grady Player May 29 '12 at 3:04
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I assume you're using a factory speedometer. If so, are your wheels factory size? Wheels being the wrong size is a sure-fire way to throw off your speedometer.

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Yeah I am pretty sure they were stock... –  Grady Player May 29 '12 at 3:03
    
There's also the question if the diff gearing is still the original one... –  Timo Geusch May 30 '12 at 15:11
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