Something I've been wondering about for a while...

[O'Borg]

As a guesstimate I'd say your economy would be about 10% of normal.

Most fuel consumption is done under acceleration. My 03 diesel golf gets 45-50+ MPG when cruising unloaded at highway speeds (70-80mph) in high gear. When accelerating from rest or low speeds in low gears, the little MPG computer in the dash reports as low as 8mpg. Figure in a lot of low-speed constant accelerating with an overloaded vehicle and I reckon 10% is a good start.