Fuel Data Center Gas Mileage Accuracy

I have a new Honda Civic Hybrid and I am trying to figure out just how accurately the electronic gauge displays mileage per gallon. Please excuse me for posting here but I didn't get much info from the Honda group so I am
trying here because Cadillacs have had electronic fuel computers for quite some time. I've always calculated mpg whenever I fill the tank. I know that the calculations have error/variation since the gas pump cut-off won't always fill the tank to the same level so the amount of gas used is off. When I compare the fill-up calculations to the electronic gauge readings, they are always different sometimes by almost 2 mpg (both higher and lower). I assume the gauge uses some kind of flow meter sensor which I hope it is more accurate in determining gas used than the pump fill-up method.
Would appreciate any comments/info on Cadillac computer accuracy which could be similar to my Honda. Also curious to know just how these gauge and sensors work. Thanks -V
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Probably the only sure way to know miles per gallon is to drain the gas tank, put in five gallons of gas and drive till the motor quits and then divide 5 into the miles traveled.
This too has a drawback. For it will give you the numbers for the weather conditions, speed, terrain and quality of the gas of that day.
don

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Alas, it is not. Best way to determine real, overall mileage is to do it over a LONG period of time and several fillups. This minimizes the over/under variations of how full the tank really is. With my Seville, the electronic MPG reading is always slightly higher than the manual calculation.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.