Flying From Grand Junction Co. to Texas I had to make an unscheduled stop for my passenger. Closest airport was Las Vegas NM. I make it a rule to never pass up gas and even though it was more expensive than at my destination I filled up. I gave the guy the card and signed the receipt not paying attention when he told me the amount. Back at my plane I looked at my receipt and it indicated he put 26.7 gallons in. Shocked of the amount, I fired up my AFS and sure enough the computer indicated 18 gallons used. I checked the fuel drains and both were not leaking and never have. In 3 years the fuel computer has never been wrong. I explained the mistake to the attendant and he blamed me stating my tanks were not full when I left Grand Junction. They were most definitely full. I did a complete preflight as most do when they will be flying over the mountains. To burn 26.7 gallons in 1.75hrs I would have been burning over 15.5 gallons/hour. At 13,500ft in a fuel injected 0-360, I do not think it is even possible. After explaining this to him he remained convinced that I was trying to rip him off. I bet the person who filled up before me got exactly the amount of fuel that I wrongly paid for. In Las Vegas NM the customer is never right and the airport manager will not return my phone call