One possibility is the noise from the leaking exhaust modified (temporarily) your driving habits.

I have run fuel computers on my cars since the Prince On Board Computer in the early '80s, and learned a lot about how driving habits affect fuel mileage. If you have a loud exhaust, you will hear the increase in power, that and possibly not wanting your car to sound like a Clampett vehicle will keep your foot lighter.

On my big Cats I always liked hearing the turbo, I would moderate my foot by the sound/RPM of the turbo. I have also driven by the boost gauge, tried to keep it under 6-8psi, rather than letting it run up over 15, knowing it is burning less fuel.

The sound of a broken exhaust is similar to the MPG gauge in the '80s gas M-B cars, which was basically a vacuum-gauge, the more pedal the further into the red the gauge went. Keep the foot light, the gauge stays in the high-MPG range, keep the exhaust quiet, the better the MPG.

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Gone to the dark side

- Jeff