Gallons per hour is completely different from L/100km or miles/hour because it fails to take movement into consideration. If one car is burning two gallons per hour and the other is burning three per hour, which is more efficient? Well you can't say. If the 2 gph car is going 20mph and the 3gph car is going 60mph, then you know the 3gph car is in fact the more efficient one, since the first is evidently doing 10 mpg and the second is getting 20 mpg.