I've always thought it would be neat if the accelerator pedal on cars had some sort of force feedback that was proportional to the amount of power the engine is putting out. That way the driver would be able to feel how hard they're demanding the car to work, and hopefully they would adjust their driving habits to go slower on steep hills, not hard accelerate out of traffic lights, etc.
Yeah! I liked seeing the "miles per gallon" meter that some cars have for that purpose. IIRC driving habits account for at least 10% of fuel efficiency losses, and by my napkin math, you could drop carbon emissions in the US by 5% if you 1. put such a meter in every car and 2. drivers heeded and learned from it.
While modifying pedals is risky, maybe you could take an OBD-II data stream and turn instantaneous power output into sound, or vibration... or lower your music volume the harder you push it...
