Definitely check it manually and find out. The trip computers on the '99 Chevy Venture van we had and now on my Monte were/are consistently 2-3 mpg off. That's just it though, over time I could rely on them by adding or subtracting what I knew the difference would be. I still always checked them manually when filling up. I'm kind of OCD that way.:p
If you're really losing almost 10 mpg, I would think there should be some driveability issues, but I'm no expert by any means.

 

Alright all of y'alls sound like good reasons. Just freaking sucks that it's this bad. Premium just went up to 3.29 tonight. Was 3.09 this afternoon. I knew I shouldve filled up when I was in town earlier! Hopefully Troy gas is still cheap

 

I hate dexcool and that brown gunk.
Gotta flush that crap out in the spring, completely.

Gas is 3.39 here right now.
I am about to cry.

 

Are you averaging over a trip, or using the at an instant. Get to 60 mph and set the cruise on a level area, you should be at 30mpg then.

 

i know the computer on my "00" is messed up. When i turn on th car, it tells me how many miles i can still go. when i start moving it says i can go 300miles on almost empty O.o

 

I think your cooling system can be a factor. How's your engine temp compared to "normal"? I just had a stuck open thermostat costing me 2-3 mpg. Engine didn't get warm enough to run as lean as it should.

 

thats the gm technology, it starts running on 2 cylinders and sometimes 1 =p

 

Do you really need to pay extra for the premium gas? If engine isn't pinging around I'd buy teh cheap gas. My Challenger runs on 87 just fine and wasn't recommended for the premium although I think the SRT's were. Just a waste of money to buy premuim I have read on many occasions if your car does just as well on 87. Just my .02

 

No I'm not using instant but I checked the instant too & it said I was getting 19 most of the time when I was cruising at 65-70. On the way back home the other day it got up to an average of 25 so idk wtf is going on

I can't remember what normal is. Yesterday though it went from between the 1/4 & 1/2 mark on the temp gauge to well over the 1/2 mark in a matter of seconds & it was only bout 20 degrees out so idk

idk I wasn't using premium for the longest time since I wasn't really hot rodding with the supercharger but once I started racing her & being harder on her I was told to start putting premium in her since that's what it said she needs. Believe me, I wish I could use regular but I've had a lead foot lately so I don't wanna risk it

 

I think Budsjlm pretty much summed it up. I also use the the heat and a/c as little as possible.
Thank god I got my heated seat working again. I only turn on the heat when my feet or face get cold now.