86 gallons per 100 miles. In terms of planning fuel requirements for a trip this form is more directly usable. In fact the metric way of expressing fuel consumption has always been in litres per 100 kilometres. The fuel consumption of 'ordinary' cars in 'ordinary' use varies between 30 and 50 miles per gallon, depending upon care and conditions. Note that all of this is directed towards vehicle movement.
Fuel consumption can arise in other circumstances.
These are referred to as 'mass rate of flow' and 'volume rate of flow' respectively and are dealt with in separate conversion calculators. Imagine a bucket of water being raised from a well which is 40 metres deep. The water weighs 1 kg. What can be altered, is the time taken to do that amount of work, or deliver that amount of energy. Putting it another way.