[EXPIOWA]

If you completely block your grill and underside of the vehicle it will become a zero emissions vehicle very soon because it will overheat and die.

It is not necessary to completely block the underside of the car to see gains. It would also be good to do it in stages so that you can monitor your engine temps under different loads and air temperature ranges.

Transmissions are very sensitive to increased heat. Even a small amount of added heat can shorten it's life.

I would steer completely away from covering any part of the exhaust. Done wrong you could trap enough heat to melt things like your interior carpet. I have seen it simply from a plugged cat on a car that didn't have an underbody pan. Heat dissapation is a huge concern in this case. The other thing you must consider is if you cover your exhaust it could cause C02 to build up and enter the cabin if it has a leak.

If you are going to do it be smart about it. If you do it wrong it could cause a fire, injury or damage to your vehicle.

If you really want better mileage get a small car. A truck will never be a high efficiency vehicle.

I installed a front and rear belly pan and have had very good results. In my case it had benefits beyond increased mileage. I have to watch my temp gauge carefully. Right now with the outside air temps near 100 I have had to remove the stock grill cover on the front of my car to ensure proper flow. Since the hot air has less area to exit my engine bay it is necessary to increase the positive pressure coming in.

Prepare to be flexible when you mod your car. The factory made sure that it would be durable and serviceable over a wide range of environments and driving styles. You are going to be optimizing it for fuel economy. There could be trade offs.