The tachometer on my 92 Honda Prelude Si works but the factory speedometer isn't whatsoever.

These are usually set at the factory? The NGK info site states 1.1 which is close to 42 mm.

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very similar can definetly bolt in there were some differences in the water temp sensor and the speedo sensor but it can work as a parts or swap engine

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Hope this was very helpful

sounds like the speed sensor

no there is no easier way if he didnt put the pin to the cable back on it must be reattached

Probably the Vehicle Speed Sensor (VSS). Cheap and easy to change (see ebay), once found! Probably on top of gearbox, to rear.

if you can charge the battery and drive it for a while then sounds like the altenator has failed and is not re-charging the battery, good way to test is to charge the battery, start the car and turn the light on, if everything is dimm or the motor dies there is little to no charge voltage, and will require altenator work

i have 91 honda prelude.and the speedometer is not working properly.what else can i find help.and look for to fix this problem

Go to your nearest Autozone or other parts house and buy an inexpensive plug gapper. I gap the Honda plugs at .045 .