Bad gas mileage. Nismo thermo too cold? closed loop?

[Indolent]

Temp gauge reads a hair below normal operating temp. Just recently put a nismo thermo on there since i started having overheating problems due to finding out my s14 didn't have one after recently purchasing it. No overheating issues afterwards, but gas mileage is worse so it seems.

Could it be that it's running too cold that it's running in that closed loop setting/too cold making it running richer? Maybe reset the ecu?
O2 just took a crap?

[ch1873857]

with colder temps. the ecu will richen the fuel to ensure proper afr. i dont see a nismo thermo causing this though. are you saying you didnt have a thermo at all? thats downright odd. if anything i would think it would run better.