ok since i got my celi in august, have have no idea as to what is typical for a celica when its 15 degrees outside. I was wondering about my thermostat. all through summer the needle would almost, but never go to the halfway point. now all of a sudden it will go up to just below summers point if im doing a lot of shifting, but if im on the highway my needle drops down to about 2/3 of the way to the halfway mark... to me this makes no sense, especially since it is a rapid change (you can watch it happen easily) and its when the engine is at a constatnt 3200 rpms or so...and to make things even more unusual, when i slow down again the needle climbs right back to where it was... watching my thermostat like this is making me nervous- when its above 30 it has done no such thing... anybody know what gives?

I would bring it to a shop and have them replace the thermostat, flush the radiator, and check the radiator fan.

It sounds like either the thermostat isn't opening up enough or there is some other restriction in the coolant flow.

The reason it stays cooler on the highway is because of all the cold air rushing around the engine, keeping it cool. And when you're in the city there isn't the airflow, making it hot. You can see it change because of how low the tolerance for the needle is, the whole gauge measures less than 80 degrees of difference.

check your anti freez make sure you have the right mix... you don't want ice in your tubes...that really really sucks really sucks