I'd stay away from pumping up your winter tires. you want as much traction as you can get. that is an easy tradeoff for me especially during winter months and in extreme conditions.
I am going to make some assumptions here as your car is very similar but not the exact same car that I have. you don't want to cover the section between the bumper and the radiator underneath. my car gets 99% of the cooling from there....literally. the front grill basically does nothing. my car has a little lip under it that catches the air and directs it at the radiator. I have been told that if this lip gets damaged or falls off, I would start to overheat.
the big thing with the grill block is to put it on in such a way that it can be easilyremoved until you know it isn't too much. I used zip ties at first and carried a pair of dikes (diagonal cutters) with me just incase. mine was fine with a full grill block. I ran for about 2 years before permanently covering it with fiberglass and bondo.
(shameless plug)
please ignore cheesy music
http://www.youtube.com/watch?v=4n6iNOxl5PA
big thing is to get a gauge. you don't know what works until then.
I would assume that your engine is pretty lose by now. I wouldn't go to a lighter oil but I honestly haven't tried it myself. it might be worth looking into.
realize that my mileage is more because I drive a lot and a lot at a time. when I don't have my daughter, it is 35 minutes at a time. when I do, it is more like an hour and change almost constantly running. that helps a lot. short runs kill mileage. the guy that drives 5ish miles to work is always going to see rough MPG but he doesn't burn a lot since he is only 5ish miles away.
if you do want to talk more about it. just let me know. I have tried quite a few things on the old cav. she has been a good little car. I might try to keep her an even decade.
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