[oldtamiyaphile]

Don't bother.

My van averages 6.7 city (16mph average speed) and on a really bad day 9.7 with a 500kg+ trailer.

If you work that out that's 0.006l/100km per kg.

Those are worst case figures because a trailer also adds rolling resistance, rolling inertia and completely ruins my aero (it's a big tall box). I also EOC normally but rarely do with the trailer.

My van had a 0.33 CD and I coast down tested the van+ trailer at 0.45CD

If I type those numbers into the calculator, I find that with a .33CD I should be getting ~6l/100km at 50km/h, and with a .45CD I should be getting 8.7l/100km. So it seems that my 500kg trailer is only costing me an aero penalty, the weight is doing little to nothing.

A lighter car will sit higher on it's suspension so could actually cost you economy at higher speeds.

So on my new van I happily fitted a heavier radio, speakers, sound proofing and even an active sub, as the comfort, convenience and enjoyment is worth the extra 0.000-0.001l/100km or so it might cost me.