E34 wins coveted award!

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   Fingerman-the brim method for measuring fuel consumption fairly accurately consists of filling the fuel tank quickly until the pump clicks off, then trickling in fuel until you can see the level rise in the filler neck.
Just make sure you avoid sharp driving manouevres for the first few miles otherwise you'll slosh fuel out the filler neck and down the overflow.
Next you either go for a very long single drive, or a weeks worth of representative normal motoring until the tank is at a low level-preferable less than quarter full, but not right on empty!
Then you just re-fill the fuel tank again to the brim, note the exact amount of fuel you put in, and then note the mileage you have covered on the odometer.
Then you simply divide the distance covered (in miles), by the fuel consumed (in gallons), and you get your mpg figure.
The only other error you may get is odometer error, but these are usually not too bad in that respect, and you can check it from maps  and known distance markers on long journeys.
The E34 has an 80 litre tank, or 17.6 gallons in old money 
From your quoted figures, 440 miles on a 17.6 gallon tank equates to 25mpg if you run the tank to almost empty before refilling, which is risky.
If we assume a safety measure of re-fuelling when the red light comes on, at 8 litres remaining (1.76 gallons-say we assume 1.6 gallons to make the maths easier), then that would mean you have covered 440 miles on 16 gallons, which equates to 27.5mpg.
490 miles would equate to around 30.6mpg, so your OBC reading of 33.6 mpg is around 10% optimistic, which is what my 540i worked out at funnily enough.
I once got 485 miles out of my 540i before the red light came on. On a recent Scottish trip I got 520 miles out of my '95 525i 24v Touring auto before the low fuel light came on.
Well, all that maths was about as exciting as watching stone-cold oil drain out of a sump on a winter's day  
Derek M5-you're naturally not biased towards the M5 in any way 
Engine sounds are very personal choices-I find it depends what mood I'm in. Sometimes I like the smooth 'zing' of the small-block BMW straight-six.
Most of the time though I love the aggressive V8 bark of the 540i at the redline-more of an 'I'm going to head-butt you' type of engine noise than the cultured howl of the 6-pot, or the aggressive cultured bellow of the M5 at the redline 
My next door neighbour will totally outdo all of us though-he has a 1970 Chevvy Camaro 5.7 V8, and the snarling racket this monster makes at idle makes my 540i, even at the redline, sound like a broken washing machine.
When he blipped the throttle and I was standing at the back of this tank, an unholy cloud of hot, sooty exhaust smoke nearly knocked me over and gassed me.
Something to do with 'rich mixture on the carburettors', whatever those strange things are 
Sadly if I had an M5 the local ***heads would try and 'borrow' it-my 540i already had 1 failed attempt to steal it from the driveway and suffered a damaged door and steering lock, which I bodge repaired.
They didn't even bother with my old blue boring 525i Touring, so there's another reason for Joe Bloggs upgrading to a BMW to avoid the 'sexy' models and get a less-desirable-to-thieves model.
Such as a 525i 24v Vanos perhaps?
See-you all know it makes sense 