After using the Fia grill cover for two years, my observations have been;
No increase in MPG that I can tell, but,
It does keep the underhood temperatues warmer while setting at night and I'm sure that it helps the electric block warmer do it's job.
And it really cuts down on the road grime that gets all under the hood during the winter months, the air filter has stayed cleaner also

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TWO Identical 2005 Liberty Sport CRDS
#1 CRD, 174,000 miles. Eco tune, Samco hoses. Mobil1 0-40w used since day one
#2 CRD 143,000 miles. Bought used (unknown history) with 100,000 miles. Samco hoses.
Eco tune, new rockers, and 5 volt glow plugs installed at 124,000 miles

I'm considering getting one once I'm back in the rocky mountains

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2006 CRD, 2nd Owner, GDE ECO, 5V steel fast-glow plugs, updated fuel-head

Bio-Diesel when/where I can. In Houston available at http://www.houstonbiodiesel.com/ and it conforms to ASTM. They also have home brew classes!!

60 % Suburbs N. of Houston, TX - Conroe / The Woodlands Area 40 % Colorado and the West

I got one for myself and now i notice that it gets up-to working temp fast with in a mile or so and the bug mesh will be good for summers.

I've ordered 1 and believe the advantage will be keeping the engine warmer & getting up to optimum temperature sooner. If it provides a boost in mileage great, but I don't expect it to.

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Dennis
Considering sound advice, invent your own solutions
CRD