k-series -
Welcome to EM! When you posted "K-series" I thought you meant the old Chrysler "K-Car" platform,
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I have the "MPG is a hobby" approach, so I am not as interested in the payback, but the "pay for itself" philosophy is a 100% legitimate way to approach it, especially if you are writing an article. Bang for the buck and all that.
CarloSW2