2000 S-10 4.3L running cold

A couple weeks ago my 200 4.3L s-10 started running colder than usual. It was running around 200, but it sits at 175 all of the time now. I've also noticed my gas mileage has dropped about 5mpg too.
What could be causing this the Truck as 58K on it and has been well maintained since I bought it 43K ago. Is it something as simple as the thermostat? The heater still pumps out plenty of heat too.
Thanks
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operating temp, she stays in Open Loop and your fuel mileage takes a dump. When an S-10 with a V-6 is at operating temp, the heater wont blow plenty of heat, it will drive you out of the cab lol. Whitelightning
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Whitelighting has a point. I have a 97 S-10 with a 4.3, after the truck is fully warmed up and your driving it, you cant leave the floor setting on too long, otherwise it starts to burn your feet, atleast it does on my truck anyway. I don't complain about the amount of heat it puts out, I just wish it would warm up quicker. Its been between -5 to 13 degrees Fahrenheit here. Coldddddddddddd I think I might put some cardboard in front of the radiator.

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Thanks for reminding me. There isnt much traffic in my area, but if i head down to the detroit area, it will be a concern. Its just of matter of me noting being lazy now and taking my grill off to put the cardboard in.

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