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...