which to believe MPG maths VS trip comp

[slindborg]

So weve done a few tank fulls of derv and the last two have been according to the trip comp(2) 44.xmpg and according to my maths 41.xmpg (the last tank having had a nice long run getting 52.xmpg from MPg(1) )

my maths being
Odo Miles/(Litres to click/4.454)

I know 3mpg dosent seem like much in the grand scheme of things, and I do know trip computers lie as I've written software for other OEM ones, but they are usually more confidence inspiring.

Or is there something wrong with the car B5.5 estate 1.9 130ps, recent air filter and 5/40 quantum platinum + oem filter change.