Manual ( and all else I read) says 18.5 gal,but I can put ~ 20.1 gal. What is the actual size?

 

It's approximately 18.5 gallons. You may just be paying more by having the extra gas going through the pump's vapor recovery system back into the gas station's tanks.


http://www dot epa dot gov/donttopoff/

 

The gas can be seen going into the car!

 

A vapor recovery system has holes in the sides of the nozzle so that vapors and excess gas can be returned to the station's tanks.

I have an 8th gen Accord, it has never been close to being able to be filled to 20 gallons.

Each time you do fill ups, fill the tank all the way, then divide the miles you traveled from the last time you filled up. This will give you your average mpg. Compare your mpg with any of the times you can fill to ~ 20 gallons of gas, and to Honda's published avg. mpg.


http://en dot wikipedia dot org/wiki/Vapor_recovery

 

An alternative, you can fill the tank to around 18 gallons or so, and try to add more gas via gas containers (since the containers won't have a vapor recovery/return etc.).