You might get an aditional 2mpg with a smart car, but I don't think your going to get serious return on your investment on your average car. Unless you have a bus or 18 wheeler where rolling resistance is a much bigger factor its not going to save you much money.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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