Ordinarily a gas tank must be vented because it's contents expand and contract with temp. changes. These gas fumes are bad for the environment. In order to reduce emisssions these gas fumes are vented to a charcoal canister, but vented none-the-less. Todays hybrids, in order to be AT_PZEv the gas tank must emit 0 evaporative emissions (liquid gas fumes). Thus the gas tank cannot be vented. So Honda makes it out of plastic so it can still expand and contract with temperature changes.
In the Prius its a rubber bladder for the same reason.

what are the disadvantages and advantages of rubber vs plastic?

i got around 20 km a liter when i drove a civic 5 spd using advanced hypermiler techniques
and now I drive a salvage nwh20 prius that gets almost to 25 km/l with 195,000 km to date.

Hi Guys:

___Let me throw in a curve ball here … Because I drive the PZEV Accord, I was worried about how much of an overfill I could get vs. the non-PZEV available in the 45 non-Clean states given the HCH-I PZEV vs. non-PZEV. Both Accord’s have a 17.2 gallon spec vs. the 11.9 and 13.2 gallon specs of the HCH-I PZEV/non-PZEV respectively. Well low and behold, my first fill was > 18.5 gallons and her real limit is somewhere around 19.6. This is the same as the non-PZEV Accord. PZEV is basically 0 evaporative yet from everything I can see under the Accord, it is a std. stainless tank by comparison to the regular Accord’s that are for sale in my locale? That is the curve ball so do with it what you must …

___Good Luck

___Wayne R. Gerdes