Sounds like maybe you don't have the correct units set for your forum signature. Go to the signature page and check the units set next to this line: "This page is showing banners with fuel economy units in...". You probably want the units set to "UK MPG".
If it is the "Fuelly Banner" you are talking about, it does take some time for it to "catch up" with the new average mpg. At least that is what I have found. I've had several fuel ups which give me a higher average, shown with the fuel up info, but the banner will not update till maybe the next time I visit the sight, then I will find it correct. Hope that helps. Lyle