sherlock

So before asking a million questions I did some reading why I am getting really bad gas miles. So i changed the coolant temprature sensor, and new air filter, and the wires and plugs are fairly new, I get only like 300miles per tank or 325 at most. My other car I get over 400 which is a 92, the 1990 is giving me problems. SO i changed the one sensor it did not help, So when I was changing the smaller one next to the one everyone recommends, it fell inside the engine bay, I do not know where it is, I am like okay no biggie, it will either fall out or just stay there, so i went and bought another replacement from autozone, now my temp gauge is hitting the next line above half?? is that because the sensor is not the same calibration as the factory one??

My rpm gauge does not work as well, I am not sure what the problem is.

1990 130k 2nd owner for about 5 years
1992 130k 2nd owner for about 6 years