I just read article in composites manufacturing about weigth and fuel consumption:
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If the link doenst work... Ford have done a test with lightweight materials and their results are that 20% weigth reduction is worth of 2 MPG.
In finland car magazines they claim in test have proven that +200 kg of weigth costs you 0.2 litres/100km... However they dont say the percentages of the weight reduction.
What I have noticed the weigth has guite big difference if you are doing P&G driving all the time. +200 kg cost me 0.4litres/100 km according to SGII.