Ah well I must disagree here, I'm sure you can do the maths. Filled up to the brim, drove just over 200 miles to Sheffield, filled up again and had used £25 of fuel. Now I tend to use Sainsbury's Super unleaded which cost me 85.9 pl but in Sheffield the same was 89.9 so a good 20p extra per gallon. So at approximately £4 to a gallon that would equate to 6 gallons. And that was definately not my imagination or incorrect calculations. Oh and it's a 1990 LS400.
This is all rather vague. And I make this between 32 & 34 mpg. Which is still NOT 36mpg!
As for those who rely on their trip computers to calculate mpg, these things are always a trifle optimistic.
I still say 36mpg in an 1800kg car with a 32-valve, 8-cylinder engine with auto transmission is just not on (silence, Maneesh!); I reckon 32 is about top whack and amazing enough, really. Especially to someone who used to run Jaguars with carburetters, where 20mpg on a run was cause for great satisfaction. (As was a month without something falling off, but that's another story.)
I promise to shut up now. I just believe accuracy is more useful as information, that's all.