I had not thought about the head/tail wind effect changing the climb but not the econ descent speed. It makes sense but in that case econ climb would be sometimes faster (headwind during the climb) and some times slower (tailwind during the climb) than the descent speed, in my experience however climb speed is consistently faster.
The price of fuel is included in the cost index and my question was about cost index generated speeds rather than generic (airline generated) descent speeds. Can and do airlines especify a particular cost index logic? I thought the FMC had just one cost index program?. Airlines just choose the number that suits their operation at any particular time. Operators who don't care about the cost of fuel just choose a very high cost index and the speeds will be very close to VMO/MMO. In that particular case I guess my question is not relevant.