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I can try to answer the mpg improvement. My baseline is 31-32mpg for my 99 Saturn SL2 automatic.
After installing the coroplast hubcaps, my mpg jumped to 34-35. I don't have any hard data, but I bet the hubcaps added 1.5 to 3 mpgs (or 5 - 10%). My drive is 56 highway, 8 city. I drive 65 - 75 mph to keep up with the flow of traffic.
I cannot remember, there's a thread on this site referring to an article that stated.
Air drag comes from:
40% shape and proportions of the body,
30% wheelwells,
20% under the car,
10% airflow through the engine bay.
I will say that I agree b/c I installed a full belly pan and I know that the hub caps added more mpg and were 100x easier to work on.
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