I think a sticker that says "By driving under 55 I save $3245 per year in gas." people understand cash really well. Problem is at the way things are going you'd need to replace the sticker every 3 days!
maybe a scrolling led light board is in order.. it would be really cool to be able to pipe your actual real time millage into a LED screen on the back of the car.. Especially when FASing!!!
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