After reading a couple of threads on testing "actual" mpg I decided to try the brim to brim method as the car's computer cannot be relied on.
On my first try I was pleasantly surprised to find that I was getting 43 mpg and not the 39 mpg that the computer said.
On the second go it only worked out to 40 mpg .
Conclusion;
The brim to brim method is no more reliable than the computer as you cannot tell by looking at the fuel level in the pipe if it is exactly the same as when you filled it the first time.
On my first try I was pleasantly surprised to find that I was getting 43 mpg and not the 39 mpg that the computer said.
On the second go it only worked out to 40 mpg .
Conclusion;
The brim to brim method is no more reliable than the computer as you cannot tell by looking at the fuel level in the pipe if it is exactly the same as when you filled it the first time.