MPG from car vs actual calculation

Discussion in 'Clarity' started by Schaffer970, Oct 24, 2018.

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  1. Schaffer970

    Schaffer970 New Member

    Took a 1087. 7 mile trip the other day (Grand Junction CO to Las Vegas NV and back) and decided to keep track of fuel used and mileage. Started with a full tank of gas and full charge. Used a total of 24.7 gallons of fuel. Was able to charge the car a couple of times during the trip and at the end of the trip charge was zero (ICE kicked in). Using the mileage and gallons of gas results in 44.04 mpg. As you can see the mpg calculated by the car is 48.5 mpg (charged the car and filled up fuel just after the picture was taken). Does everyone see this much of a difference between actual and what is shown on the car display? Still very happy with the mpg but wonder about Honda's calculation. 20181022_175630.jpeg

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  3. glockgirl

    glockgirl Member

    I have not seen a big difference between mpg tracking apps and Honda computer. The only time I was able to keep track of HV mog is on long drive where I wasn't able to change at all. I was getting about 38-42 mpgs.
    On days where I deplete the EV, 90mi days, computer shows I'm getting about 53mpgs.
    I don't have a problem with that. I normally get close to 2000 on single fill up. 2k on 7g tanks is way better then anything else on market.
  4. dpuhrmann

    dpuhrmann New Member

    I have seen similar results to Schaffer970 where the calculated MPG is overstating the actual MPG. Went on a trip that was around 280 miles and when I did the math, I came up with 54.67 MPG (still AWESOME), but the car stated 59.5 so it is off about the same amount as Schaffer970.

    NOTE: I have yet to get the HV mileage fix done on my Clarity (waiting for my first oil change to knock everything out). I wouldn't think this should impact the calculated MPG, but wanted to throw it out there just in case there is a correlation.
  5. ClarityDoc

    ClarityDoc Active Member

    Displayed MPG is pretty meaningless if you're adding energy via charging, since the "gallons" aren't providing all of the "miles". Multiple prior threads have discussed this challenge.

    The Clarity doesn't segregate ICE miles from EV miles (and this is understandable since the traction motor is electric and the ICE is basically a generator). It's a shame - I'd like to know mpg; more importantly, if the Clarity doesn't "know" ICE miles then it's hard to estimate need for oil change.
    KentuckyKen likes this.
  6. dpuhrmann

    dpuhrmann New Member

    Not sure I understand this? The car shouldn't need to know when the ICE is on to calculate MPG, it only needs to know how far it has gone and how much gas has been used. I understand that this will mix in miles that you are using EV, but still should be able to calculate it. For example, you go 280 miles on ICE, 40 miles on EV and use 7 gallons of gas, the MPG should be 45.7MPG.

    I totally get it that the car can't tell exactly how much gas it used to get ICE only miles, but it should be able to easily calculate how many gallons were used to get a certain distance even if gas wasn't used for some of those miles.

    My assumption is that if the car were trying to display the MPG for ICE only usage, it would be well below the actually mileage you got on that tank of gas + EV miles... What we are seeing is that it is higher than reality.

    What am I missing :)?
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  8. Schaffer970

    Schaffer970 New Member

    This is exactly my issue. It's hard for me to believe that the car doesn't know to a high degree of accuracy how much gas is being used. Adding energy via charging will alway skew the number up but the value shown on the car should be the same as calculated by the owner - total miles divided by gallons.

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  9. jdonalds

    jdonalds Well-Known Member

    This really isn't an easy issue to deal with. It seems that if the engine is running one would only need to know the fuel consumption during that time-frame. However some energy may be going straight to the motor driving the wheels while some power may, or may not, be going to the battery. Later that battery stored power can be used to drive the wheels. Even when the clutch is engaged and the engine is directly connected to the wheels it may also be sending power to the battery.

    When using the manual method at the pump again it seems straight forward to just take the gallons used and the miles driven. But not all of that can always be attributed to the gas engine because the battery may have been externally charged, or regen could add power to the car.

    As always the car toggles between gas engine running and battery power only, or a combination of the two.

    So the car isn't running independent of external power (where I also include regen). It isn't like an ICE car where all power can be traced back to the gas.

    I really don't know how to compute MPG, so I gave up trying to do it. I'm quite happy to know, with a solar roof on our house, we drive for free most of the time and I don't care if I get 42 mpg or 50 mpg on occasional long trips. The car is great either way.
    Oak Hill Dan and insightman like this.
  10. PHEV Newbie

    PHEV Newbie Well-Known Member

    I'm the same way now. When the car was new, I was obsessive about calculating the HV mileage. When I consistently beat the EPA estimate by a long shot (even after factoring out the few EV miles expended), I just fill it up and drive as a hybrid when I need to. As with others, I discovered not to drive in HV mode with a depleted battery (keep at least 60%). That way, the car gives you maximum power when you need it and delivers the best gasoline mileage. With a depleted battery, it can be a terrible experience. That's why the Clarity has gotten some very bad reviews. I noticed that reviewers often drive the car with a depleted battery.
    Vezz66 likes this.
  11. Schaffer970

    Schaffer970 New Member

    I guess my real question is how the car (Honda) is calculating MPG displayed on the dash. If it is not total miles divided by gallons used what is it? I agree that the number may not be of great importance (it will get larger the more electrons added) but as an indicator of efficiency it would be nice to understand how it is being calculated.

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  13. KevinW

    KevinW New Member

    Can you provide some more details on this? Brand new Clarity owner so I haven't yet experienced much driving outside EV mode.
  14. insightman

    insightman Well-Known Member Subscriber

    There are other threads where people go into great detail but here's my quick take.

    The Clarity's 103-hp internal combustion engine (ICE) is not by itself capable of spirited motoring because it's trying to power a 4,059-lb car. If you drive with a depleted battery, the engine works so hard that the revs rise dramatically, causing what people on this forum call the "angry bees." It's unpleasant to hear and is likely responsible for many of the negative reviews uninformed car testers write.

    However, when the ICE is just supplementing the power coming from the battery (HV Mode), it can remain a low-key power partner to the 121-hp electric motor. The ICE is very quiet when used this way. When the ICE and battery are working together the total comes to 181 hp (it's complicated why it's not 121+103=224 hp).

    If, like me, you mainly drive under EV power and are careful to never let the Clarity start it's ICE (we went 10 months this way), you'll be surprised at how much power you have in HV Mode.

    In HV Mode, the Clarity PHEV runs mostly on ICE-power as it works to keep the charge in the battery from dropping. After you use some battery power to accelerate for passing or to climb a hill, the Clarity in HV Mode uses regenerative braking or, possibly, some engine power to restore the battery to the state of charge it was at when you pressed the HV button.

    An unfortunate decision by Honda was to cancel HV Mode when you turn off the Clarity. That means on a long trip you have to remember to press the HV button every time you start the car. I made a paper HV button to tape over the D button to remind me.

    Unlike HV Mode, HV CHARGE Mode is to be avoided. The Clarity PHEV gets its worst possible gas mileage when using the ICE to charge the battery while it must also provide power for driving. The "approved" use of HV CHARGE Mode is to increase the charge in the battery before driving up a mountain after using up most of the charge on the last mountain.

    It appears my quick take wasn't very quick after all.
    Last edited: Oct 25, 2018
  15. PHEV Newbie

    PHEV Newbie Well-Known Member

    If you're in HV mode and encounter an upgrade, with a low charge on your battery (doesn't even need to be depleted), your ICE will race loudly to high RPM to generate electricity for the traction motor. Because the gas engine only produces 103 horsepower, it's insufficient by itself to power a 4054 lb car uphill at highway speeds. The car's computer doesn't know how long the grade is so it won't draw from the high voltage battery unless there is a major charge. If you have a major charge, it will combine the electricity from the generator and the big battery and you can keep up with any car in the fast lane. The engine hums pleasantly in that scenario. The better mileage is anecdotal but explains why some reviewers get amazing mileage and others don't (e.g., Car and Driver getting 46 mpg on a 75 mph loop, about the same as the much smaller Prius Prime under the same conditions; Consumer Reports got much lower mileage but it seems they did their testing with a depleted battery because they complained about the noisy engine and poor performance). It also explains the fantastic mileage reported in this forum vs. some very poor mileage reports. I've consistently gotten between 48 - 52 mpg in HV mode keeping a charge above 50% (subtracting out the few EV range miles expended). Of course, I stay within speed limits so I'm not surprised I do better than Car and Driver. By watching the power distribution in HV mode, I noticed that car can go for miles in EV and then recharge those miles when most efficient (flats and downhills), while other times locking up the clutch. With a depleted battery, it seems to do a lot less because it is harmful to the battery to expend electrons when the battery is so low. By restricting what the system can do with respect to energy usage, it's not surprising that mileage is impacted with a depleted battery.

    Although the Clarity can drive with a depleted big battery, I don't think Honda designed it to be used that way. Otherwise they would have given us the large 2.0 liter ICE engine like in the Accord Hybrid, which weighs almost 700 lbs less than the Clarity but uses the same drive train outside ICE size. There is certainly enough room under the hood for it. Think about it. A fully charged traction battery can give you maximum 120 hp and combining the traction battery with ICE electricity generation gives you a maximum 181 hp out of the traction motor. That's only 61 hp by adding the ICE generator. It's unclear how many horses you can get if the only electricity was coming from the ICE generator but it won't be much more than 61 hp. If you're cruising in the flats and never need to pass anyone, that might be plenty but you're going to be severely horsepower limited in HV mode with a depleted battery. You would think that the car can draw from the remaining two bars but it doesn't seem to be programmed to.
    Oak Hill Dan and KevinW like this.
  16. akcoffee

    akcoffee New Member

    Interesting... Historicaly, Honda trip computers underestimate MPG: my 2007 Civic Hybrid (now the kids' car) is typical it under reports by 3%. When I drove it I'd just add 1.7 mpg to the number on the dash to get darn near the actual result.

    Prius (maybe Toyotas in general??) over estimate by 5 to 7%. My 2014 plug in Prius lies to me by 7% and my wife's Prius V lies by about 6.

    Btw not a Clarity owner, yet.
  17. KevinW

    KevinW New Member

    Thanks insightman and PHEV Newbie, great answers. Sounds like the takeaway is that if you know you will be doing a lot of highway miles, switch to HV mode early in the trip while the batter is still halfway full. This would also mean that if you take a trip out of town then you need to keep it in HV mode while at your destination (assuming you can't charge at your destination) so that you have sufficient battery for the drive back home.
    insightman likes this.
  18. Viking79

    Viking79 Well-Known Member

    Driving with a dead battery and resetting the trip meter, my car seems to overestimate fuel economy by 5 to 10%. It says 44 mpg and I gas up and it is 40 mpg, as an example.
  19. M.M.

    M.M. Active Member

    Very small nitpicky clarification: The electric drive motor is 181 hp, not 121 hp. The battery pack, however, can only put out 121 hp (90 kW), so if the ICE isn't running that's the max you get. With the ICE adding power (via the generator) you can get up to 181 hp from it to the wheels.
  20. insightman

    insightman Well-Known Member Subscriber

    Thank you, you are correct, sir or madam. I should have caught that when re-reading my post. Apologies to anyone whom I confused with my error.
  21. Vezz66

    Vezz66 Member

    More importantly from a standstill, do we get full torque with only the battery?

    Stats here
  22. petteyg359

    petteyg359 Well-Known Member

    Related, the stupid "gas mileage" in the navigation estimates gallons based on miles traveled (clearly not actually linked into the fuel pump). It estimates _all_ miles traveled as gas, so tells me I've used now than twice as many gallons as the car has had put into it.
  23. insightman

    insightman Well-Known Member Subscriber

    I've wondered about that ever since the first specs were released for the Clarity PHEV. Horsepower numbers were given for with and without the contribution of the ICE, but only one torque figure, 232 lb-ft. I would assume an electric motor's torque as well as horsepower are proportional to the amount of energy flowing into it, but my understanding of electric motors leaves much to be desired.

    I know the i-MMD system varies the voltage delivered to the traction motor to increase efficiency, but there has to be a cutoff, below which the torque must drop off. Perhaps the battery power is always great enough to exceed the cutoff and additional power from the ICE doesn't result in greater torque? Doesn't seem right, but my understanding of the i-MMD system leaves even more to be desired than my understanding of electric motors.

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