Quote Originally Posted by Diesel_vert View Post
The density of premium unleaded petrol (95 RON) is approximately 0.75 kg/L, so the difference in mass between a full tank and half a tank on a Mk7 Golf is 19 kg.

Assume a full tank is filled at $1.40/l, while half a tank is filled at $1.49/L (taken as the average of $1.40/L and $1.58/L). That is a price increase of 6.4%.

Therefore, fuel consumption must decrease by 6.4% in order to break even. Whether that will occur from carrying 19 kg less mass, I leave up to you to find out.
Yeah this looks right, vw claim that a passenger can increase fuel consumption by 0.2l/100 so this would in all probability be almost negligible.