Smart tires idea: How to measure electrical output of piezo arrays?
A 1.25x1.5x0.02", 15nF, +10.23mm deflection, 1.77 N/m stiffness, PZT 5A, Piezo Electric Harvester, two layer plate costs $33.00 each, per 100 units, and generates 17mW Power and 5mA at 8g 420Hz vibration, which could be enough to power most RF electronics devices at the tire.

A standard tire costs $150 and last 3 to 4 years.

For generating 1W per tire with 60 units, good for most power consuming electronics with 3.3V and 300mA available, for a very small LED light, and insuficcient for anything else, like car lights and other systems, and assuming all the mechanical parameters for the piezo device are met, and ignoring inefficiencies, the main reason for not using piezo tires is the final cost. 

Raising the power rating to 1W will require $2000, and a whole set of 5 piezo tires would cost the same than an old used car.

Normally this should be a deal breaker the use of piezos for anything else than RF sensors, but a high end car focused on some key safety product as added value would use those piezos if they can be naturally integrated without problematic issues into the manufacturing process.