I think I figured out the payback as follows;
If the Kicker,bracket,etc cost $3000 then:
Total cost + interest(any time you use $ for something it incurs an "opportunity cost", b/c the $ ,even if it's not borrowed, could have been "invested elsewhere with an alternate rate of return) =$69/month for 4 years (@5% annual rate)
So that $69 x 12 months = $828/ year
$828 / 30 trips(conservative) = $27.60/ trip (this would be the break even point)
I can't see how the kicker's gonna save me $27.60/trip since that's more than it cost to run the I/O for the whole trip!. Hard to figure in savings on maint/repair on the I/O.Anyway, the point is that "on average" a kicker is not going to pay for itself in fuel savings until it's paid for, which of course, is a contradiction in terms.