Ha ha, no! It's my job! - hence Steve mentioning in another thread I was on 178,000 miles at the last GTG a few weeks ago! [find out my current mileage by clicking on my sig!]
Sprintmonitor is interesting - I looked up some Vito 120s on there and confirmed that mine is in the right ballpark as far as overall MPG goes (low to mid 20s mpg).
A friend of mine recently got one and is apparently getting almost 40 mpg on the trip computer - I told her it must need re-calibrating!
I joined that site and after fiddling around getting to grips it tells me I got 15.6mpg out of mine yesterdy/today, which is near enough the same as my car says! (Not that I'm pleased with that sort of mpg mind!!)
Any measurement over a single tankful will be meaningless.
The attached graph shows the consumption of my CLK320 Cabrio since I bought it in August. Every fill of petrol was to full as indicated by the first auto cut-off of the pump.
The variation from 23mpg to almost 32mpg is due to variation in driving from complete tank-full used non-stop on the motorway to all city stop-start.
Only now after some 6000 miles are we getting to a reasonably accurate figure of approximately 28mpg.