Running a little cool?

Lately I have noticed that my 2003 Grand AM Gt has been running a little cool. Cooler than normal and my mileage per tank has dropped by about 20%. I am not sure if the two are directly related but if there is a way to easily diagnose the problem of the car running cool I would do that. The temprtature here right now is cold (between -2 and +10 Celsuis) but I would think that the engine would still warm up to its normal running temperature after a while or driving, but it never seems to get there. Is there anything that I can check myself to see why it might be running a little colder than usual? The gauge on my dash for temp read from 40 (low) 90 (mid - where it should be) and 130 (hot) but it never seems to get past about 70 I would guess.

Thanks for any info

Welcome to AF !

When its realy cold it might take a while to get warmed up and may never reach running temp. when its below zero.

Probley your thermostat, if its the original ones Id try changiing that. Make sure you get the 195 deg thermo since it gets so cold where you live.

You can also take a small piece of cardboard and cover part of the radiator. If it still doesnt reach temp then its your thermostat.
Just dont forget to remove the cardboard when it warms up outside.