If you are trying to replace a faulty speedometer, there should be a couple of clips on the back of the instrument cluster. All you need to do is push the lever and pull the connector out. It is illegal to disconnect the speedometer in the United States for any reason other than to repair or replace it.
Check the fuses. Sometimes you have to remove and reinstall them.
A sensor.
No.
No 2003 was a new refresh on the Neon, the shape and curviture are different.
the gap is .035 for a 2003 neon
According to fueleconomy.gov, a 2003 Dodge neon gets 19 mpg in the city and 27 mpg on the Highway. The annual fuel cost of a Dodge Neon is about $2000.
coming to a complete stop
nope
The speedometer is located on the instrument panel directly behind the steering wheel and is typically located in the center.
The battery is under the hood on the driver side in a Dodge Neon.
The PCM on a 2003 Neon is between the radiator and the transmission under the hood.
Depends on what's wrong with the speedometer. Try re-soldering the contacts on the mainboard behind the instrument panel.