RPMs vs. speed

1998 Ford Taurus
3.0/6Cylinder
128000

Is there a way to find out what the RPM gauge should show at certain speeds? Example would be, at idle mine is showing between 500 and 1000, so is that where it should be? When I am going say a constant 40mph what should the RPM's be?
I think it should be between where you discribe at idle. It should drop about 150-200 rpm'swhen you place it in gear. The rpm's at 40 mwill vary somewhat dependant on the gear selected, surface you are driving on, temperature of the engine and transmission. Generally it should be around 1800 to 2200 if you are warmed up and running on a smoot level road.


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