There are different types of hybrids. The trick is in when you use what energy. The hybrid I propose uses a small to medium size combustion engine(same as in current vehicles - established conventional technology) operating off natural gas with a small auxiliary electric motor and battery pack. You run the vehicle on natural gas normally.
When driving in the city or accelerating say for example to pass another vehicle the electric motor kicks in on a universal drive system to compensate. These driving conditions are where your biggest emission problems occur. Starting and stopping frequently and accelerating.
You only use the electrical motor for short distances in the city and when accelerating. The combustion engine,when used, runs at its peak operating point constantly - further improving efficiency. The key is targeting the biggest inefficiencies in operating conventional vehicles.
The equipment cost and weight hurt you more than it helps. Rip the $7,000 batteries (aren't those going to fun to replace in 8 years) (and motor/generators) out of today's hybrids and both (H & T) would get just as good mileage, probably better. Cost per mile would drop big time.