I installed a MMW header and custom 2" resonator back exhaust with a Walker muffler about a month ago. Install of the custom resonator back exhaust was done by Emmanuel at Emmanuel's Mufflers in Alameda, CA. (http://www.emmanuelsmufflers.com/). The only other mod I have is the AEM (dealer version) CAI. I used to have a DC axle back exhaust which was a bit loud for my taste. The current set up I have now is slightly lower in tone with no drone at freeway speeds. Still a little loud for my taste, but no unbearable. It's a matter of personal preference, and this set up works for me.

Prior to the exhaust install, the best MPG I could muster was about 31 MPG with primarily freeway driving. I checked my MPG after the header and exhaust install and was a bit surprised, so I ran few more tanks of gas just to make sure it wasn't a fluke. I filled up today and averaged 35.47 MPG. This is again primarily freeway driving, but including stretches of bumper to bumper traffic Monday thru Friday.

The increased mid-range torque and stronger high RPM pull were great benefits in and of themselves. But with gas prices pushing $3/gallon, the added MPG was a pleasant and expected gain.

Andre

i was thinkin the same thign today when i was driving home from work but then i realized that i already get about 36.4 miles per gallon driving 50/50 highway and city. if i new it would be higher id consider the same but it wouldnt pay itself off yet

You guys are lucky I get about 27mpg in my XB. Then again I drive 75-80 on the freeways here in LA. That would be about 3600-4000 rpm. Oh well I guess.

heres some personal things i do about driving to save...

1. when going over 60, use ac and roll windows up
2. drafting a semi helps, but dont get too close to cause overheating
3. do the speed limit, it helps
4. fill up gas at night, gasoline is more dense when its cooler
5. go to more popular stations to fil, fresher gas
6. keep tires at constant 30 psi, less roling resistance
7. ride in lowest gear in city when possible

heres some personal things i do about driving to save...

1. when going over 60, use ac and roll windows up
...

I always use more fuel with the AC on vs windows down.

Mythbusters did a test using SUVs I believe where they put the same amount of fuel in 2 trucks and ran them around a track at 50MPH or so. One had the windows down and one had them up with AC on. The one with the AC ran out of gas long before the one with the windows down did. Every vehicle will get different mileage but the difference was significant enough to say it the AC that used more fuel.

yeah i remember that epiusode. but it also goes into how the ac system is designed with the car. cause i mean 3500 rpms at 70 mph for an xa/xb wouldnt put much strain on the motor as its already under a load. also goes into the desiogn of the car and how much drag is put on the vehicle when the windows are down. with my old thunderbird it only had two windows and the air would pocket in the back and cause a lot of noise. cars with more windows or those with a hatch window that opens are helpful cause air can flow freely thru the car. in my moms kia sorento when i go on the ihighway i open the hatch window and get excelent mileage no ned for ac