Gas Mileage is dropping

[MRDANBLANK]

I changed out my thermostat last weekend. When I removed the old thermostat, it broke into 4 pieces. After replacing the thermostat, I let the truck warm up for about 15mins. Ever since then, I noticed that it takes about 15 mins of driving for the temperature gauge gets close to the standard operating temperature. It tends to fluctuate throughout the first 15-20mins of driving. Even if I let the truck warm up for about 10mins before driving.
I've also noticed that my gas mileage has been dropping ever since I changed the thermostat. I was thinking about unhooking the battery for about 20mins to reset the computer. But I'm not sure if that's going to do anything. I figured I would see what you guys had to say about it.
Thanks in advance.

1998 Ford Ranger XLT
2.5Liter 4 cylinder
approximately 150,000 miles on engine.

 

[BrooklynBay]

Did you replace the thermostat with a low temperature model?

 

[dogfriend]

Make sure you installed the thermostat the right way. Spring side goes toward the engine. The small bleeder hole goes toward the top. I installed it backwards on my 89 Ranger once and it would take forever for the engine to warm up. This will kill your mileage too.

There is also an outside chance that the new thermostat is defective, but not likely.

 

[MRDANBLANK]

Thermostat

I replaced it with a high gauge thermostat. I think it was a 190.
I put it in the same way it came out. I know I did, because I had to pull the top of the old one out with a pair of pliers.

 

[BrooklynBay]

What brand was the thermostat? Did it have the bleed hole with the check ball? Some thermostats are designed as a "fail safe" meaning if they fail, they stay open.

 

[MRDANBLANK]

What brand of thermostat?

It was a Advance Auto Part. I believe it was the Stant.