[JQmile]

I did this a couple years ago with my Dodge Ram and drove from Vegas to LA and broke 30 mpg. The grille block alone was worth about 2 mpg at highway speeds. I wouldn't worry about engine temps, the new trucks run 230 degrees all day long and they're the same basic engine. Also with the part throttle conditions you truck will be under it will only need to make about 40 hp to propell the vehicle, so I wouldn't be worried about an efficiency drop--you'll only see a big effect under full power. If you do tons of highway driving and don't tow or go up a lot of hills I'd leave it on. Just be mindful of those engine temps if you're in the throttle too long. Here's the 30mpg dodge 1-ton article link:

1997 Dodge Ram 3500 Dualie Cummins - Diesel Power Magazine