Got fed up with mine and bought a Scanguage II and it's bang on for accuracy as long as you enter the correct amount every time you fill up, a few seconds well spent IMHO.
Cant say ive heard of the scanguage, any good?
http://www.scangauge.com/products/scangaugeii/Here you go, have to say I've been very impressed by it, I bought mine after Lincolnbaggie recommended it, I think one or two others have had them too