Driving slow in the carpool lanes, even driving the limit, is very rude as since it is a single lane, peopel don't have the option of (legally) passing until they get to openings, as a result, a single slow driver can cancel the entire advantage of the lane for everyone behind. I have no right to force the drivers behind me to be more fuel efficient by driving slower than the flow of traffic in the single-lane carpool lanes, or in the passing lanes.

The biggest offenders I see though are not hybrids (and there are many I've seen in the lanes near here, always going with the flow of traffic) but rather slow buses and vans from Mexico that decide to go 10 under the limit in the carpool lane...

Whenever I have used the carpool lane, I've simply followed the car in front of me at a reasonable distance, as everyone should do in a single-lane environment like carpool lanes. If those drivers are going too fast for my taste, and the regular lanes are moving, I need to stay out of the carpool lane. When traffic is moving though, I don't generally like the carpool lane as it makes it more difficult to move over to exit the freeway. At times when traffic in general is congested, the carpool lane is crowded enough that it's going below the limit anyway. That's a situation where it's vastly more fuel-efficient ot be in the carpool lane, as it's a steady, efficient speed, rather than an erratic, slow speed with lots of stop and go.

Another fuel-economy benefit of the carpool sticker is bypassing the metered onpams which release cars one-at-a-time. This means I don't have to stop then accelerate up to highway speed on the ramp, I get a "running start" which saves a lot of gas.