An Imperial gallon is 20% larger than a US gallon therefor take the Imperial milage in this case 36- 20% =28.8 mpg. its that simple .

Tony:

The US officially adopted the Metric system during Teddy Rooseveldt's second term, I believe (1912 or 1914? ). The ANSI and NTSC (otherwise known as SAE for society of Automotive Engineer) "inch" standards were promulgated later. Before that time, nearly all fasteners were hand made by blacksmiths, and you got what you got....

I am fairly certain I don't what to know the reasoning, I've had enough for today!

Peter