How about an URL printed on the back of your car that people can go to from their smartphones and pay for your gas? In return you go faster.
Maybe a tag that looks something like this:
__________________
2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)