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Speedometer Accuracy?



My speedometer, and while i am on it, the tach are acting kinda funny.

My question is

How do the speedometer and tach run?

The speedometer bounces a lot and occasionaly the tach will bounce up abou 200 rpm while the engine is actually steady.

If i changed the rear gear, in the differential, for instance from a 3.08 to a 4.11 would i have to adjust the speedometer for it.

Usually the tach measurement is taken off the distributor. The faster the engine goes, the faster the distributor has to spin to spark all the cylinders, so there is a direct correlation to engine speed and distrubutor speed.

The speedo takes a measurement off the rearend of the car. There is a set ratio between the rear end gears and the diameter of the rear tires. If you change the size of the rear tires, but leave the gear in the rear end that runs the speedo unchanged, then the speed reflected by the speedo on your dash will be incorrect. If you increase the size of the rear tire, your speedo will reflect a slower speed than what you are actually doing. And if you decrease the size of the tire it will show you going faster.

The same is true with changing out the rearend ratio, only opposite. A higher ratio i.e. 4.10's would read faster than what you are really doing, and a lower ratio would read slower than your actual speed. It is possible to get the speedo recalibrated when the new gears are installed, but I dont know what it entails.

my '81 speedo did the same thing....bounced around. i found the problem at the cruise control box...the cable was loose there. when i screwed it tighter it worked perfectly. as for the tach...if it's electronic, you may have to rebuild it or replace it.
but, i have to agree with twiget....if you changed the rear end, you have to make some gear changes in the speedo cable.
there is a plastic gear in the tranny that the speedo cable runs off of. This gear can be changed, I think they are identified by color and/or number of teeth (obviously the main difference). The problem is it's not obvious as far as I know which gear to change to. There are speedo shops in most big towns that can calibrate a speedo after a drivetrain change.

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