Fuel saving.

Following on from Sunday's race, I have some questions and thoughts on 'fuel saving'.

As we are always told, the teams under-fuel their cars to save on weight as apparently each lap of fuel adds around 1 tenth of a second onto a lap time. This bit I'm fine with, carrying extra fuel slows you down - no problem there. However, towards the end of many GP's now, a lot of the drivers are told to go into fuel saving mode by turning the engine down, short-shifting and letting the air resistance slow the car down instead of just the brakes.

Thing is though, how much potential time do drivers lose by fuel-saving compared to the time they'd lose by carrying extra fuel so that they could drive faster for longer?

Is it because the teams hope that by the time the final pit-stops have taken place, their drivers are in their predicted positions meaning that they can cruise to the end of the race? If this IS the case (which I suspect it is), it is totally pointless having the races as long as they are. If the first 75% of the race is a sprint but the remainder of the race is a amble (so to speak), what's the point of the last part of the race?

I'm sure that the teams have very powerful computers making incredibly precise calculations on fuel requirements so my points may be worthless but it all seems a bit strange to me.

Surely there must be a way to enforce a ban (as well as possible) team-directed / pre-planned fuel saving. You could check the engine's fuel requirement when the driver is on maximum attack (ie. fastest lap during qualifying) then calculate this backwards to the amount of fuel needed at the beginning of a GP assuming the driver will constantly be driver the b******s off the car. I know it isn't quite as simple as that but the principal remains true.

Any thoughts on the topic of fuel-saving?