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At WOT it sounds like you have a 4bbl card. During normal driving, it's not that much louder than stock. When you start to accelerate, the sound becomes progressively more noticeable as more throttle is applied.

I did a subjective test tonight on external noise level:

I had someone on the street while I drove by in two different modes.

1. Normal - Driving down a quiet street at 30 mph and slowly accelerating to 40 mph. The noise level was just slightly louder than stock. Nothing that will make the neighbors run to look out their windows.

2. Full Throttle Burst - 30 mph to full throttle for 2-4 seconds. No mistake that something is coming down the street! No, it doesn't sound like you're at the tracks. As one observer noted; "Just like the old days - sounds like a quad at WOT".

BTW, I also have a 5zigen Fireball on the backside. Although it's not that loud during normal driving, it does add to the noise level at wide open.

Unless you have a "lead foot", the noise level is acceptable under normal driving conditions. Anyway, the stock IS is not a "quiet" car compared to the other Lexus models. The stock exhaust sounds a little more aggressive and there's more tire noise.

Gas mileage - Still under evaluation.
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