Well... I judge my mods not by how much more power I "feel", but rather by how much kilometers I do on a tank of fuel (I judge a good mileage to be around 400km+, mostly city driving). These are the best ones that increased my mileage (or helped to keep it at a good level):
1) NGK Iridium spark plugs + 8.5mm performance wires from Aurora. Better idling RPMs right after the install.
2) New OEM fuel filter - incredible how much crap was in the old one and how much the change helped engine RPM at idle. A drop of about 200 RPM was noted right after the installation.
3) An always clean MAF, "NAPA Gold" air filter and new Bosch O2 sensors (front banks only - I was told rears don't do anything for fuel economy and just measure cat performance).
* Recently I also deactivated my failing DRL module (after putting in an HID conversion, 40% less power drain than OEM bulbs, but more light!) and now drive with lights off in the day. I don't have any data yet, but I'm sure the reduced charging load will increase mileage too. Worst case (if law enforcement starts getting upset), I'll install some LED lamps in the front of the ex for daytime driving, but so far I don't judge it to be necessary.

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And the worst mods? I think they deserve to be listed too:
- The "Vortex" generator for $20 + shipping off ebay - in fact just a long & flat piece of cut aluminum sheetmetal with fins (that you have to bend and fit into your TB yourself, lol). Didn't do anything, looked ultra-cheap and I removed it before a piece of that crap would break off and damage the TB.
- Fuel additives/other additives that come in little expensive bottles and claim to solve all your life's problems. Besides using an occassional injector cleaner, I believe they're all a load of crap, like liquid bandaids in a hemmorage situation. No results - no mileage or power gains, ever... and I know some might try to put a contract out on me for saying this, but that includes
Seafoam! Ran it in my fuel, cleaned the engine with it through the brake booster as suggested... and nada, no increase in anything. I'm actually lucky it didn't foul my expensive plugs, as some members have had happen by using it on their engines.
- Anything else that claims to be doing something magical that no one has ever thought of before, despite that the automobile's been around for over 100 years. Especially if that something comes in liquid form!
