Re: gallons of gas. There's the old puzzle: your spouse gets 100MPG in that super-hybrid-mobile. The salesperson wants to upgrade you for $1000 to the super-duper-hybrid-mobile at 200MPG! Double the mileage!
You suggest instead that you get the old truck serviced and replace the plugs, distributor and tailpipe. Estimated cost $1000, and should get you from 10MPG to 11MPG. Which is the better deal? Assuming you both drive about 100 miles per week.
Isn't it precisely like that? The point of the exercise is that even when you are getting really high mpg changes (e.g. 100 to 120), the best gain is improving the the really slow component of the pipeline (e.g. the truck from 10 to 12).
