Most mods especially to the engine don't increase gas mileage. That is because more HP means more gas. If you allow more air into the engine the computer sees this and adds more gas to keep the fuel air ratio where it has been programed to. You can free up the engine and maybe get a little more MPG such as changing the pulleys but after figuring in the cost of the mod parts and the small MPG increase it may not pay for itself for a long time. Other than keeping the front end properly aligned, upping the tire pressure a little, the biggest factor is your driving habits. In the June 2008 issue of Consumer Reports page 46 they give some things that will help. Listed are, Driving at a moderate speed, Drive smoothly, Reduce unnecessary drag, Don't use premium gas unless your car needs it, Minimize driving with a cold engine, Keep tires properly inflated, Buy tires with lower rolling resistance, and Avoid long periods of idling. They went on to say that a myth is a dirty air filter will lower gas MPG. This was true in the old days. Now the computer just reduces the gas amount to compensate for the less air. Noted in the section under driving at a moderate speed is this. Driving a Camry at 55mph vs 65mph saved 5MPG and driving 55mph vs 75mph saved 10MPG. I think this was done with a 2006 Camry which has a smaller engine but our cars should see the same kinds of gains. I have been doing this for a while and I can see much improvement in my gas mileage.
we installed an injen cold air intake and a magnaflow exhast system on our 05 tC and we've been getting an extra 3 miles to the gallon...not bad. It probably won't pay for itself in the long run, but at lease it improved instead of getting worse. We haven't made any modifications to the xB yet.
I have the INJEN CAI and it may or may not add MPG. I will never know, because it sounds so good when I put my foot in it..................IM getting around 225 a tank wich suxs, so I changed my habits for 1 tank anf got like 280 so driving habits are a must in adding MPG...