Number of factors can cause the lowr miles per gallon - among the most common, low air pressure in the car's tires.
Look at lower rear of engine
upper or lower?
The 2001 Honda Civic OBD 2 port is on lower right edge of dash; behind console trim
The 2001 Honda Civic OBD 2 port is on lower right edge of dash; behind console trim
at the top of the lower rad hose (in housing)
Thee 2001 Honda Civic OBD 2 port is on lower right edge of dash; behind console trim
You can find the size of the lower alternator mounting bolt on a 2005 Honda Civic on civicforums.com/forums/21-i-c-e-electrical/354635-diy-alternator-replacement.html
The thermostat on a1998 Honda Civic is found on the right side of the engine. It is located in the thermostat housing which can be found by following the lower radiator hose.
In the Honda Civic line up, the EX monicker typically denotes the top-end trim package. Other common monickers include LX and the DX which refer to trim packages offering fewer features but at a lower base cost.
Follow the lower radiator hose into the housing, that where the thermostat is.
Should be on the lower rear of the block, near the dipstick.
The 1991 Honda Civic featured changes over the previous models such as increased dimensions and a lower hood line. You can get more information about this at the Wikipedia. Once on the page, type "Honda Civic" into the search field at the top of the page and press enter to bring up the information.