I think I paid $7-8 for a 9ft foam rubber seal, plus I had to buy two rolls of heavy-duty double-sided tape.

I got my through a buddy who works at a sign shop. It was free as it was cut-offs from a larger project.

As for the plugs, they will mostly give you greater durability over the stock xB plugs, so you don't have to change them every 30K miles. You might get a tiny power & MPG boost, but don't expect anything dramatic.


Oh, I finally changed the front O2 sensor & re-gapped my plugs to .044. On a ~70mile drive where I had previously gotten 42mpg (on the scangauge) *at best*, I managed to average 47.5mpg without trying nearly as hard. So I am inclinded to beleive that with the O2 sensor making the engine run better I'm truly seeing the benefit of the undertray. As of now the scanguage says I'm at 38.5mpg for the tank (and only the last ~70mi is with the new sensor), but I'll see when I fill up as the SG tends to be optimistic.

Alright. Thanks! I hope the tape sticks in all weather conditions! :p

By and large it is sticking ok, the only problem spots have been the slight "kickup" towards the center of the bumper on each side, and the very back edge near the tire.

The "kickup" may only be because it poured rain right after I installed it and water got in between before the adhesive on the tape had set. At the back edge the lip on the bumper for the tape to stick to is small and the seal gets pelted with water & dirt from the tire. I'm goign to peel some of the tape off and re-stick it, and add a few screws to reinforce it when I get the chance. But it is staying pretty well thus far.

Did you replace both o2 sensors? or did you just replace the front one? I ask because I have been told the rear one lasts longer. Not sure why.

I just installed NGK iridiums gapped at .044 and the car idles much better. I have not run through a full tank yet but I am interested to see if it helped.

Guess I need to get a scangauge soon so I can see what is going on between fill ups.

Or some clamps or something could work. That little indent is an issue, hmm?

Just the front one for right now. The rear one tends to last longer since the exhaust has been treated by the cat by then and is cleaner.

They are a pretty sweet piece of kit.

Well my tanks since the addition of the undertray & lip have not shown any improvement. If anything they seem to have gone down slightly.

I'm wondering if the front lip is to blame. There is a plastic piece under the bumper that acts a bit like a front belly pan, and I'm wondering if blocking that with the lip is actually worsening air flow there.

I think I'm going to pull off the lip and try that for a tank or two, then maybe try a smoother front belly pan & possibly a partial grille block.

yes take off the lip. Those are there it the underside airflow sucks, but your airflow is good so it is just hampering you.

When doing the measures, have you guys calibrated the ScanGauge? Mine was way off optimistic (>10%).

Else use it to compare before/after mods, and not for absolute MPG readings. Don't adjust it until you have results without the lip.

If the lip lowers MPGs, then maybe just put a dam in front of the wheels to deflect air from them without adding frontal area (since drag = Cd x surface).

Now that the undertray/lip will almost be tested, maybe you can start looking above for spoiler impact on MPG.

I use the scanguage for a rough idea of my mpg while driving, but figure my mpg per tank the old fashioned way. I fill up at a lot of different gas stations so due to the variance in pumps I can't ever get the SG dialed in 100%.