I asked Ant Davidson a very similar question, today, over twitter:
is it more detrimental to a car to be fuelled for whole race and run 100% than to underfuel and save fuel for some of the race?
sometimes,but like always it depends on how your race unfolds. Sometimes it actually boils down to the tank not being big enough.
Sorry I can't give any figures. I can't remember the figures but each kilo is worth a 1/10s or so. It's not that much of a reply but then Twitter doesn't lend itself to proper answers.
"It could be done manually. It would take quite a while, but it could be done. There is however a much more efficient and accurate way of getting the data. Men with lasers." Wing Commander Andy Green