I just had my fuel pump changed, oil changed and new air filter put on my 2000 chevy blazer. I have 1/4 tank of gas left and I only have gotten 180 miles so far. Before the changes I was getting around 290 miles to a full tank for city driving, why the sudden and large drop in my miles per gallon????

I would do all the typical maintenence stuff that affects gas mileage before I consider any real problem (assuming you haven't already). When was the last time you did a tranny and differential flush? After I flushed mine my gas mileage skyrocketed (my mechanic said fluid was black). Make sure tire pressure is good on all four tires. Clean injectors lately, maybe add some Techron at next fillup? How old is fuel filter?

You could also be getting the same gas mileage and have the gas gauge be off.... my gauge has been off since I had the fuel pump and sending unit changed, not sure why. What to do is run the truck until the low fuel light comes on and then fill her up, see how many gallons you put in and you will know exactly if its the sending unit and gauge or if it needs to be tuned up.

I will check the tires today first. The fuel filter is about 1.5 years ago, I don't do a whole lot of driving, mostly to work and back. I don't smell or see gas so I don't think any is leaking anywhere. I will try the additive also cause I need to fill up anyways, also will take note of how many gallons it takes to fill up. thanks for the info.

Ok, so my engine light came on yesterday, went to auto zone for diagnostic.

Definition:
Fuel tank pressure sensor condition

Probable causes:
1. Fuel tank pressure sensor defective
2. Check connector and wiring
3. Check hoses and fittings

troublshooting code P0452

Sounds like issues with either installation or part from the fuel pump, no?