here'* a mind boggler for you

Im getting 15-16 mpg, and this isnt one of those "i just got home from sunoco and i got 15mpg, what do i do" situations. haha this is something thats been happening for about a month or two.

alitle background info, 107000 miles.
October 2006 FRAM airhog airfilter. (replaced today with a K&N)
January 2007 @100,000 miles it got a tuneup . Plugs, (never bosch) wires, belts, oil. New June 2007 Tires properly inflated
July 2007 High Flow catalytic converter - Transmission serviced.

Im still getting 15mpg with all the upgrades. the FRAM had 13,000 miles on it and thats nothing for those filters. it was a year old and i changed it today with the K&N box filter and gutted the box...it feeeels awesome. lets hope that helps. But at one point it was getting 22mpg around town.

Most of the driving is local but the bonneville always got 22-24 mpg. i dont have a heavy foot, i learned the hard way when the green bonneville died to be a little less rough with my engines.

What do you all think it could be? o2 sensor beings its probably original? and are there any warning signs of that going or is this it? and how many and where are they located?? filllllll me with knowledge.

Could be the O2 sensor

or this...

lol thats for when im in the Buick Grand National we have in the garage

dad has his classic..... im waiting for pontiac to bring back the Trans Am

any thoughts anyone?

I'd start with the oxygen sensor, and see what that does. At your mileage, it is definitely due for replacement.

I second (or third ) the O2 sensor.

Also, it could be the gas you use. In the summertime, oxygenates are added to the gas to lower some emissions. This will also decrease your mileage (which means you are burning more gas and making more pollution - smooth move you politicians, eh). Another thing to watch for is alcohol in the gas. This has to be one of the biggest boondoggles also. Do what ever you can to avoid it.

I definitely say o2 sensor, not a whole lot more to decrease your mileage than that. Also, check tire pressure (nvm if you have a tpms) and you should be good to go.

Also, hawkjet, you've got that a little backwards.

Summer fuels have much lower levels of volatile chemicals actually increasing the fuel mileage quite a bit, but these chemical compounds, known as MTBE, ETBE, and ethanol this lowers the evaporation rate in summer temperatures, they ALSO contain many less hydrocarbons (propane, butane) than winter fuel, making it a much cleaner fuel. This nice summer blend makes a much heavier, non evaporative fuel. In the winter time...these aforementioned oxygen-bearing lightweight compounds displace the normal fuel components, so it takes more fuel to get the job done. Normal summer gasoline has a stoichiometric ratio of 14.7:1, reformulated winter fuel runs between 14.3 and 14.4:1. Therefore summer fuel, less power, more mileage, winter fuel, more power, less mileage. A nice thing is that in the winter, cold temps mean dense air, and a nice fuel to supplement that never hurts, so therefore cars get significantly more peppy in the winter, but turn into gas guzzlers when you start enjoying that pep.

Yes, ethanol is something that isn't too great, while having a very high octane rating, it decreases mileage simply because it doesn't have the same BTU rating as pure gasoline, or gasoline with many less additives, so basically you're using more for the same effect, pretty much putting you back to the mileage you would get in the winter, which basically defies the original purpose of burning less fuel, saving fuel, while, in a very small scale, burning a cleaner fuel.

i had a feeling it could be the sensor

1. how many do the 3800 2'* have.
2. where'* that link on BC showing how to service the o2 sensor. (i know there'* always a link )

and maybe a website to buy it from, or am i going to Pontiac

thanks everyone

2, one post converter and one pre.

you can buy them both at rockauto. only buy denso or delco.

they are very easy to replace, just unscrew and unplug and then install and plug in.

Hold on a second..

Only replace the front one...unless you want to spend some money unnecessarily. The rear O2 merely monitors the clean exhaust to make sure it'* clean. Because of this..it'* not exposed to the same levels of exhaust the front is and is not prone to failing like the front.