labrat

It's probably closer to about 24-25kg in weight saving, since the S.G. of petrol is about 0.8. However, in principle you're correct. I did an exercise in basic physics the other day. I wanted to know what effect the mass of the vehicle had on the power required to achieve the same acceleration performance (say seconds to 100km/hr). It turns out that the power required is directly proportional to the mass of the vehicle. That means for example, if your car weighs say 1350kg, and takes 6.5secs. to get to a 100km/h with 177kW, if you put an extra 50kg of mass on your car, you'll need an extra 3.7% increase in power, or about 6.5kW extra just to maintain the previous performance. From my reading of the performance threads in this forum, this is about as good as most of the claims for the bolt on bits that people have tried.

The lesson is: save your money, and keep your car lean and mean (and keep yourself slim and svelte).