rhk
What's the best way to adjust my speedometer? It constantly reads 5 or 6 mph too high.
paget
This is a common problem. In fact, it always was a problem but most people ignored the fact that the speedometers read non-linearly (that is, they were correct in certain ranges and not in others). There's nothing you can do in this situation. However,if your speedometer is consistently reading 5-6 mph too high the chances are you have the wrong speedometer drive gear in your transmission. You need to get the proper gear for a specific rear end ratio. Another way to "fix" that sort of problem is to change the size of tire, but it involves some calculations. A third way to fix the problem is to take the speedometer apart and bend (ever so carefully!) the indicator arm.
tjbian
If it were a gearing problem it would be a percentage variance, not a constant error at all speeds. My only experience is with Buicks. The driven gear on all ratios is 7 teeth. My 40 Super had 4.4 gears, the driven gear is 22 teeth. I changed the gears to 3.9 gears from a Century. I now need a 19 tooth gear to make the speedometer read correctly. 

Your needle may be dislodged. An indication might be if it was not at zero at rest. In this case bending the needle carefully might correct the problem.
tjbian
Since my last post I was able to solve my speedometer error problem by purchasing an adapter unit from Texas Industrial Electric

 http://www.texasindustrialelectric.com/speedo.asp

They asked me to drive a 5 mile measured stretch on the Interstate using the mile markers and record the odometer readings. In my case 5 miles showed 4.1 miles on my odometer. I did two tests to be sure. They produced a unit that has a gear train and mounts to the side of the transmission by the fitting for the speedometer cable. The speedometer cable is then inserted on a similar fitting on the unit. The unit cost about $150 including shipping. I now have a correct reading speedometer.  In my case in addition to the rear end gear ratio change I am also running larger tires so the 19 tooth gear would still have given an incorrect reading.

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