> What else could there be? Charge a fee based on the number of ppm's your car emits: tax * ppm = fee to renew your tags Even better would be to look at

> What else could there be? Charge a fee based on the number of ppm's your car emits: tax * ppm = fee to renew your tags Even better would be to look at the odometer reading each year: tax * ppm * miles driven last year = fee to renew your tags