|Even a dachshund could look smart!|
The trick is very simple: just convert vehicle mileage from miles to the gallon to liters per 100km. Use your brain only and aim for a precision one digit after the decimal point. For example, try to tell how many miles to the gallon is 4.3 liters per 100km. For a greater effect, refrain from moving the lips, facial expressions and moving of eyeballs too often.
Words of warning: if you have absolutely no idea how to do that, please refrain from cursing as it would have an effect opposite to the one you are after. The trick won't work if you are one of those who could calculate the orbit parameters of debris after Hail-Bop crash on the Moon without using pen, paper and team of PhDs.
The trick will work for a while, then you will get better in this so no life warranty here. Thank me later!