I was getting around 12-13mpg earlier this year, i replaced the air filter and boom, my mileage went up to 17-18, i was pretty happy because the car drives fine and throws no codes, so the whole summer i got good mileage, then about a month ago, my mileage took a hit, how big i didnt know till i filled up. i went about 170 miles on 13.4 gallons!! thats about 12.5mpg. What the hell could it be to take that big of a hit, now i do a lot of short trips and car doesnt warm up always but still that cant be it at all! Even on the highway it took a hit, Keep in mind before this i replaced everything and it turned out to be the air filter, i doubt the air filter is that clogged after 3 months. its a paper fram.

Mike

 

Driving habits...perhaps?

 

The worst part is out of the 170 miles i got, 90 of them were pure highway which makes it that much worse!

 

I had a o2 sensor go bad and had similar drops in my mileage.

 

my first guess would also be the O2 sensors, check some other stuff too, fuel filter , spark plugs , maybe you just got a bad tank of gas too ?

 

that's a pretty significant drop in a short period of time. Usually you will see a gradual drop if a fuel or air filter is the problem. So my guess would be bad O2's or a spark plug went bad for whatever reason. O2's are costly tho, so you don't want to be buying new ones if they aren't the culprit.

 

Well see i have 2 brand new o2s in the box because last time my mileage took a hit, i thought it was them, but the car isnt showing any codes or misfires! nothing. Only thing i did was change oil to german castrol! and get a coolant flush. Everything else i checked from when i had this problem before i i changed out the air filter.

 

If you've got two new o2's then put them on. Also, until the car warms up you will get worse mileage, it's part of how these cars are tuned to run when cold.

 

Well i do take short trips and sometimes the car doesnt fully warm up, however 12mpg is cause for something not right.

 

Maybe....but not necessarily. If the temperature where you are is relatively cold then the short trips/car not warming up could explain the poor mileage.

 

But the thing is, when i noticed the hit, it was warm out which was over a month ago. as ive had the same tank of gas for over a month since i went racing, i just filled up yesterday, i knew the mileage was crap, but when i calculated it, i couldnt believe it fell so much. ITs 50s sometimes 60s, a month ago it was 70s 80s. so its not like its freezing cold

 

im storing the car on november 15th, im gonna do some driving to eliminate if it was a bad tank of gas, i doubt it but oh well.

 

Depending on where you live in the country, there are different blends of gas for winter and summer. I know the restrictions were lifted for a little while after Hurricane Katrina, but I'm pretty sure the restrictions are back in place. Where I'm at in Maryland, I'm pretty sure they've started shipping winter blend gas. We can always tell the switch because of the drop in gas mileage. (And it happened to both cars so I doubt a mechanical cause.) The WS6 went from averaging 18-20 to 16-18 and the Firehawk dropped from 20-22 to 18-20 (mixed city highway driving in rush hour traffic for both cars).

Once spring gets here and they switch back to the other blend of gas, we'll see the gas mileage increase again. They supposedly switch blends because of some emissions related crap...

 

I live in Montreal, Canada, i wonder about the winter gas as well, i know back in philly same thing. but i still got at least 14-16.

UPDATE. When i fill up, the gauge always go past the full line by about a 1/4 inch and by the time it hits the first line, i have about 30 miles or so normally with city driving. Well today i goto the gym, and the stupid gauge is just a bit under the first line ALREADY!!! with 10 fking miles on the odometer! Im going for a record here. Im shooting for 9-10mpg on this tank by the looks of it!

 

I average about 26-28 mph (mostly highway). Something is wrong if you only get 10 mpg.

 

I get 19 with my mods. Even with the winter mix Gas. I would replace the front O2 sensors. If not that take it to a reputable LS1 shop.

 

Just thought of something. Reset the PCM and try that.

 

Also have your codes scanned. AutoZone does it for free. The light on the SES could be out and you are not seeing the code it is throwing.

 

well it threw one about a year ago for a bad cat, had that replaced. Maybe another cat is going out! i dont know! how long can a ls1 sip 10mpg before it throws a code?

 

Your dash light behind the SES could be out and not showing the code. Get it scanned.