Is THis Normal?

My 1990 Buick Century Limited (3.3L) is starting to seriously GUZZLE GAS and there is absolutley no reason why. would the 16" rims make me get 108 miles per tank? I know about a month ago I was getting almost 450 miles per tank but last time i ran out of gas it was only at 108.3 miles and my 15 gallon tank was probably dryer than ever.
I will appreciate any help I can get with this one.

Check for a vacuum leak. Have you noticed any other symptoms?

Increasing the wheel diameter, and the wheel circumference, will play havoc with the speedo (cause it to read low) and the mileage counter (causing it to rack up a lower count), but unless you doubled the wheel circumference (not possible on a 90 due to wheel well size) your "mileage" isn't going to drop that much.
Assuming the "check engine" light isn't on and the car seems to run OK (have the trouble codes scanned if the light's on), an O2 sensor failing can cause the fuel mileage to quickly go bad. check tailpipe while car is running, any black smoke? wipe out the tailpipe end (was it all black inside?) and run it a few miles wiping it out again. there should be very little carbon built-up after just a few miles. black smoke or bad/quick carbon build-up can mena bad O2 sensor. also check the intake around the air filter to be sure no little critters got in and packed the air filter housing full of nuts or nesting materials (personal experience with that one). good luck and hope yours doesn't eat front wheel hub/bearing assembly like mine does every 20k miles...

Many things here

vacuum leaks, O2 Sensor, leaky fuel injectors (this one has MFI), exhaust system backpressure, etc., MAF sensor

suggest checking for vacume leaks, doing a good fuel system diagnosis, checking codes, checking or replacing O2 sensor as well as plugs, filters, and other items associated with a good maintenance tune up.

I was sorta thinking my PCV may have something to do with it but this phucker doesn't have one or at least one that i can find. I know they are made on these cars cause i bought one and have yet to install it. but this is a list of all the things i have replaced.
knock sensor
crank shaft sensor
O2 sensor
Plugs
Plugwires
oil system flushed
new computer (believe it or not these cars drive better with a wagon computer in them of the same year and engine type)
but I've parked it and pulled out my K5 blazer because this is the FIRST 6 cylinder that i've ever seen to get 9-11 MPG and my K5 gets 13 with a 350 smallblock and a 4 barrel quadrajet and pushing 36" superswampers. SO SCREW THAT CAR!!!!!