I'm seeing anywhere from a 2-5% degree of pessimism from the onboard computer as well

 

First tank through, computer read 46.0mpg, I figured 49.3, so mine seems to be showing a bit pessimistic (albeit I don't have very many data points yet).

Just from the practice of the first tank, the computer is showing me at 54mpg so far on the second tank, this is fun ^_^

 

This topic comes up from time to time.
https://www.greenhybrid.com/discuss/...-mileage-9963/
https://www.greenhybrid.com/discuss/...s-actual-9427/
https://www.greenhybrid.com/discuss/...han-dash-7593/
https://www.greenhybrid.com/discuss/...s-actual-7435/

My lifetime "calculated" mpg is 56.4 mpg. My lifetime fuel economy taken from the fuel display is 55.8 mpg. This gives a difference of 0.6 mpg or about 1% for me.

 

Is the "calculated" in quotes because your odometer is off? I seem to remember a previous post to that effect. My FCD was 3.7% pessimistic about the mileage on my recent road trip, but my odometer was 2% optimistic regarding the miles according to my handheld GPS...

 

It's my HiHy that is way off (~3.5%--odometer low). I only did some very quick tests in the Civic with a handheld GPS and it was within 1 percent--but the odometer was high.

I put it in quotes because using the odometer and gallons put in is what most people refer to as calculated. The display fuel economy is a calculation as well. In my HiHy, it turns out that the display fuel economy is much more accurate than what most people refer to as calculated. I think the best way to determine your fuel economy is by calibrating your odometer and then using gallons put in (pumps are periodically calibrated as well).

 

That makes sense. This was the first time that I've used the GPS for more than a few miles. I only logged two sections of the trip but came out to a variation of 2.0% for both 150 mile sections. I wish I would have written down more of them to see if it is a consistent 2%.

I was kinda bummed about a fudge factor of 2%, but I guess it's better than 3.5%...

 

Just an observation while on a 1600 mile trip which included many miles through the Appalachian mountains.
One possibility is the on board computer is doing crude averaging of the instantaneous MPG displayed instead of averaging fuel consumed over the trip odometer. So if you have a lot of 100MPG driving like cruising down a long hill. The computer will average a lot of 100MPG samples instead of the actual mileage you may be getting like 125 of 150.

 

I don't think that is the case. When using the glide, the computer climbs very quickly on short drives--a lot quicker than throwing 100mpg into the equation. I have gotten the computer over 100mpg many times which wouldn't be possible if it were just doing a crude average of the iFCD.

 

Just a note that the graph and Excel sheet will be unavailable for a couple of days while my telco stops tripping over itself to change my service.

 

I'm pretty sure that is not the case too. The HCHII will display FE as high as 150.0 mpg. However, it stores FE much higher. For example, if you are going down a long steep hill and reset your trip B, the display FE will max out at 150.0 mpg (or it will display --- until you finally start consuming fuel). After doing this it can sometimes take a while before the display fuel economy drops below 150.0 mpg.