Now at Istanbul the weight of the fuel for each lap is worth 8/100ths of a second of extra lap time, just under a tenth. So one kilo of fuel, which is what Vettel is supposed to have saved, would give him a speed disadvantage of 5/100ths of a second over Webber in pure fuel weight.
I think we modelled it on this originally then reduced it. Anyway, according to this 1kg of fuel at Turkey was worth 0.05 per lap in 2010. Converting that to iGP litres gets us 0.0381s per litre in lap time.
This means, at Turkey, the difference of 230L in Andrew's example, should be a whopping 8.763 seconds
per lap. So it's about 4 times too low at the moment. If we simulate this realistically the tyres will need to be revised completely too.