I let my 98 Civic ex sit for. couple weeks and when I went to restart it, the tach was about 1k rpm lower than it should be throughout acceleration and cruising. But I notice that it will go to normal sometimes while operating. Also, when accelerating the transmission seems to be in too high of a gear and I get poor acceleration. It has affected my MPG as well. Now on a full tank in only getting 220 miles but used to get around 300 miles per tank. No engine codes thrown. Please help.

 

Swap in a known good ICM to see whether problem is resolved.