20000 miles in one year at 600approx miles to a tank = £2500 in fuel = GTD
20000 miles in one year at 300approx miles to a tank = £5000 in fuel = GTI (After a 4 year saving in fuel I could buy a used mk6 GTI if I wanted one).
Most people I know with a GTD average about 45mpg, I average about 35mpg in my GTI.
In a GTD 20000 miles @ 45mpg @ £1.40/litre = £2828.62
In a GTI 20000 miles @ 35mpg @ £1.36/litre = £3056.91
So the GTI costs an extra £678.29 per year in fuel, not sure where the £2500 comes from.
... i have looked at your figures and cannot figure out how you calculated them

20000/35 = 571.4 gallons = 2600lt*1.36 = £3,536 for your golf

Most of the posts on this forum suggest that a GTI only gets above 30mpg if you drive it like your granny. And most GTD drivers seem to be getting above 50mpg unless they drive like mad men. My GTD is averaging 52mpg.
Even if we just look at your figure and my figure...
My GTD 20000 miles @ 52mpg @ £1.40/litre = £2,453
Your GTI 20000 miles @ 35mpg @ £1.36/litre = £3,536
I love my GTD it has lots of smiles per gallon
