How Accurate is your GOM?

Same situation for me. I filed a complain with Honda on this about a month after I got the car. No response.

 

For me, it is about 12% off - on mpg shown vs actual mpg. I don't track miles-to-empty.

My previous car, a Civic Si, was almost exact. Prior to that, I had a Fit - it was off about 5%.

I'm still trying to get my ScanGauge calibrated to where it shows actual mpg accurately. It's getting better with each fill-up.

 

12% is how optimistic Honda’s mpg calculation on a FL trip where I kept track.

I posted about it, pondering if it was intentional fraud.

 

Ours is pretty accurate. I've verified it many times using the same route day after day. This is for EV mode only.

 

Every car I’ve ever had that estimated range or tracked your actual MPG overestimated by ~10% compared to what I measured with the odometer and gas pump. I always assumed this was intentional to make the owner feel good. Like a scale calibrated to read your weight 10lbs too light.

 

This is my experience also. I assume the trip meter is pretty accurate and with EV estimates I am usually within 1 mile plus or minus the distance on the trip meter--unless I drive over 65 mph or accelerate faster than necessary. I drive pretty carefully with a feather touch, light braking, etc., and that is the key to good mileage with my Clarity and three Honda hybrids before it. All summer I have consistently gotten 60+ miles estimate for EV and travel that distance before zeroing out. The best so far is 64.6 EV miles.

 

EV is pretty accurate, so long as driving technique and conditions are consistent. It’s the post facto gas mileage calculation that’s suspect.

If the car knows you went 200 miles and burned 5 gals, it should be trivially easy for it to return exactly 40 mpg. If it says more than 40, it seems it must be intentional.

 

It's just an estimate. This car doesn't have any visible way to separate gas miles (or km) from EV miles, so "mpg" is a fiction unless you use gas pump and EVSE measurements, odometer miles, and some creative estimation. Notions of "fraud" (post above) are bluster - these are guesstimates, and any 'HV' miles will include some 'EV' miles - and vice versa. This car has no exclusive power modes.

 

This ^^^.

 

I have not tracked closely enough to say for sure but that's because I have never expected any range predictor to get it more than just in the ballpark. Or at least within the parking lot of the ballpark. As for EV/HV maybe I'm looking at it too oversimplified but I always took it to mean that if for example it is currently displaying 50 miles EV and 200 miles HV, it is saying that it estimates that if you drove EV from this point forward you can go an estimated 50 miles before it reach 0. Or if instead you drove HV from this point forward you would go an estimated 200 miles before you ran out of gas. Not sure that it's trying to do anything more complicated than that. It bases each estimate on previous EV and HV driving. How good the estimate turns out to be depends on the specific drive that you are making at the moment.

 

I've found the Trip MPG to be off by 10% favorable. It uses HV and EV miles for it's calculation
Recent long trip where I used actual gas usage. Trip Length 996 miles real trip mpg 49.48 vs the Trip MPG calculated by Honda of 54.4. Factoring out EV range drop from 60 to 22.5 the HV MPG would be 47.62. That is one fantastic MPG for a car like this. Did a similar trip in the winter and got about 38 HV MPG so I'd have to say cold weather (mid 20 degrees F) really saps range in HV as well.

 

That's an interesting point. All cars get lower mpg during winter for a variety of reasons, however a hybrid has additional losses because it uses the battery to store and retrieve energy and that is less efficient during cold weather. So for a PHEV it's not just driving in EV mode that is affected during winter, but also driving in HV since even then the battery is still in constant use.

 

It’s important to differentiate between forward-looking estimates and post facto calculations.

It’s the latter where it sure seems Honda adds about 10% to its gas mileage calculations. Other manufacturers do it as well, virtually always overstating the actual gas mileage obtained. Just seems suspicious, is all. “Bluster” notwithstanding!

 

I have never owned a vehicle from any manufacturer that returned better actual results than those calculated by the computer, so leads me to believe it's by design.

 

With all my vehicles, I just plan on starting to look for gas when I get to 1/4 tank. With only a 7 gallon tank in the Clarity, I may start looking at 1/2 tank.

 

Not me... I just drive until the "low gas" light comes on.
That is the beauty of this vehicle - If you run out of battery, you have gas. If you run out of gas, you have electric !
Also, when the low gas indicator comes on, you will still have a substantial number of gas miles available in spite of the GOM that some pontificate about...

 

From an emergency/safety standpoint yes it is nice that we can run out of gas and keep going at least as far as we have EV range. However as in any car it is still best to avoid running out of gas because there is a chance of damaging something. The chances might be small but not knowing I try and avoid it.

The owners manual also says not to run of out fuel, although I suspect they copied the text from their standard owners manual (notice the reference to the "needle")

NOTICE
You should refuel when the needle approaches E.
Running out of fuel can cause the engine to misfire,
damaging the catalytic converter.
The actual amount of remaining fuel may differ from
the fuel gauge reading.

There is a forum member who ran out of gas one time and they describe the error messages that resulted, although apparently no harm was done. Also in their case the fuel didn't actually run out until 15 miles after reaching 0 HV, which I would guess would probably be true in most cases, although the actual distance beyond 0 HV would of course vary depending on the situation.

Daniel M W
Jul 6, 2019
My wife and I had a bad situation and we ran out of gas once (but the car still had around 20 miles of electric range). So a few things happened:
1. Low fuel light turned on, and we keep driving until all lines in the gas gauge disappeared and the HV range showed 0 miles. After that we drove for around 15 more miles.
2. The car was on HV mode so when we run out of gas, the ICE motor stopped and a message appeared in the dashboard: "power system problem"
3. After that, the car kept running on electric mode, and when the electric range dropped to 15 miles the message changed to "power system problem, stop driving when safe"
4. Thank God, we reach an open gas station when electric range was 8 miles. We put 7.3 gallons on the tank (0.3 more than specified on the manual). After filling up, I turn on charge mode and the engine started normally, then everything back to normal.

https://insideevsforum.com/community/index.php?threads/at-what-level-do-you-gas-up.6111/#post-67993

 

I am certainly not advocating running out of gas.
Rather, my point is that you are not risking running out of gas if you wait until the low-fuel indicator comes on.
All modern vehicles have the low indicator function and they are designed to give you ample time to get to a gas station safely.

In the case of the Clarity, here is the excerpt from the manual:



When the light comes on, you have "approximately" 1.1 gallons left. In almost all cases, you would have no problem getting to a gas station before running out of gas (~40 miles available). If you happen to be driving in a scenario where gas stations are few and far between, then don't wait for the light.

 

I don't know if I see much of a difference at least in basic concept, most cars have a low fuel warning that comes on prior to the gauge reaching empty, usually with plenty of miles to get to a gas station. Plenty being a relative term of course, having a different meaning in the city as compared to being out in the middle of nowhere. Also pretty much all cars that I have ever driven or ridden in went past empty and kept running for a certain number of miles when you got into that situation inadvertently.

Clarity's fuel warning may come on earlier than some other cars, and yes I agree that makes it pretty safe to wait until the fuel warning comes on to refuel while in the city. But my point is the fact that Clarity can continue to operate after running out of gas is not something that I factor in at all when thinking about when to fill up. Although it is good to know that if I mess up I likely won't be stranded on the side of the road, although I still risk at least some chance of causing some damage. And I'm not sure that any damage especially to the catalytic converter would be immediately apparent. But yes it is true that running out of gas in Clarity has much less risk of causing an issue with personal safety.