Here are some pictures of what I'm talking about!
Its not mounted on or under the rear bumper or under the motor.
It wraps the transmission way before the engine compartment!
Prevost diagram

Right side looking towards rear from in front of transmission while under the bus!

Left side looking towards rear from in front of transmission while under the bus

This also goes up to the bottom of the floor area and to my understanding it reduces the air pressure that is created without it in the entire engine compartment and thus allowing the fan to draw in fresh air thru the radiator easier! Without it, a higher pressure is evident in the engine compartment forcing less air to flow thru radiator. With it, the air is deflected downward and exiting under the bus and outward exiting to the sides resulting in less pressure in the engine compartment!
I feel if it's good for 5 degrees or 5 mph, its time well spent!
Total cost about 100 bucks! The mud flaps were 20 bucks each but any good plastic or rubber would work!
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