From current answers the good basic size would be be 155/80R14 + option for XL load rated version. That would fit easily to most cars here and the diameter would be 603mm or 23.74 inches. I think there would be only little gains to be had for going even narrower. Also what I have learned little bit wider tire gives better rolling resistance figures also.
In europe "truckers" (these F-250 etc...) are so little minority that there would not be any sense to make eco size there.
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