Getting horrible gas mileage on our 2018 Honda Clarity, 25-28 mpg!

Well, it hasn’t even been 24 hours since the first post.

Sure we’d all like more info, but I don’t see it as a waste of time to try to answer someone’s questions and try to help them better understand their vehicle. It is a quirky car that doesn’t just get 42mpg and 47 EV miles under all conditions.

From the looks of it, there’s about a dozen of us who don’t really have any pressing matters to tend to, so maybe we can tough it out and see how this resolves.

 

It has been more than 48 hours, but your point is well taken !

 

I actually can believe what you are experiencing but could be for a couple reasons. I have hit 28mpg once myself. However, temp was right about 0 degrees Fahrenheit and was driving though icy passes. That is the only time I have hit that low.

I have a 2018 touring with 43k miles. Live in northeast Washington state. I used to calculate the mpg by hand when fueling on long trips like people suggest but found the calculation by the car was within 0.5mpg as long as I reset when changing to hv so rely on its calculation.

Cold definitely affects the mpg as does speed, amount of heat used, tires. Lowest 28 as above. When new in summer driving 800 mile round trip with some city driving, freeway at 75mph averaged about 44-45mpg. Summers now on freeway about 41-43 mpg affected by speed (going 65-80mph). Winter trips typically about 35-38 mpg. All through mountain passes. Commonly taking the car skiing right now driving majority of way 50-60mph. Averaging 46.7mpg.(that excludes time driving in electric when I reset meter- I drive down my hill all in electric, up the steepest part of the ski hill electric and repeat in opposite order on way home with the middle flatter part in gas).

First thing I would consider from your report that you can fix regardless of driving style. Do fully charge the battery or at least have it 3/4charged even if you are going to drive in HV. Have seen up to a 5mpg difference (usually about 2-3) driving on depleted battery vs full.

2) heat is a huge player. When I drive EV I get about 40 mile range in winter, wife gets about 28. Difference is I set the heat at 67. She sets it at 86. Similarly I get higher mpg in hybrid then her. My understanding is unless you have the Canadian edition you are using straight resistance heating regardless of engine running or not. Not very efficient heat. (Canadian has heat pump that is slightly better)

3) tires matter. 1 see about a 1-2mpg difference (and similar decrease in EV range) as soon as we put on snow tires regardless of temp. Similarly under inflated tires has cost us 1-2 mpg as well at 1 time)

4) mode matters a little - Econ seems to get 1mpg more for me

5) finally though if you are not cranking the heat and getting those type of mpg, need to get it looked at. I have had a couple times that driving on trips, battery 3/4 full in hybrid on the 55mpg roads that I got down to mid 30mpg. At that time, no matter what I did with the regen the charge going down hills would barely be below the white neutral line. Seemed like something was wrong with battery or regen system. Dealer could never figure out why but has happened 3 times. Don’t know rules for you state and probably not as generous as for Washington, but here it follows California laws. Battery has 150k warranty and regen system 100k (I believe on the latter) transfers to any subsequent owner. I would recommend you get it checked by a dealer that knows the clarity (only one guy knows it at our shop and he LOVES it so we get good care. Nobody else there has a clue)

5) finally, if the other owner abused your battery it could be the issue. Ie if they never charged it and drive it purely hybrid. If driving up fairly steep hills with depleted battery, based on the way ours drives coming up our hill in that condition, I suspect you can force it to deep cycle and damage the battery. When we were looking, dealer had one on lot that they NEVER charged and initially did not know how to. Drove horrible in hybrid. When we drove one that was getting charged, was beautiful. Both <200 miles on them.

Hope something in there helps.

(also separate thought. Clarity phev is the best car I have ever owned by miles. (2 prior camrys, Honda Odyssey, 2015 leaf and cheaper cars for comparison). Not perfect but really nice. However, would not buy new again for one reason. Never crash tested so insurance premiums are ridiculously high. I am pretty sure it is safe but apparently insurance does not think the same. (New price after rebates/no sales tax was about the same as paying 24k with taxes for something else)

 

I can attest to the unreliability of the guesstimates shown on the driver's display. This past weekend we did a drive of about 45 miles roundtrip, about 30 of those in HV mode on the freeway. We started out with an HV range estimate of 286 miles. At the end of the roundtrip, the estimate was 284 miles.

 

Another reason insurance companies might not like the Clarity PHEV is due to the high cost and difficulty of effecting repairs. Many Clarity PHEVs have been totaled despite the apparent damage being minor because of unapparent damage to expensive electrical systems.

 

In cold weather, below freezing, the BMS could limit regen.

 

Thanks. For a moment I thought today was Tuesday.

 

unfortunately know that one from personal experience. Neighbor was distracted and drove into the front of our car when parked when just 9000 miles on it. Took a while to get parts and cost about twice as much to repair as her car of about equivalent cost and damage. (But insurance rate has stayed the same since)

 

The time I was referencing was about 70 degrees outside. Yep aware of the regen in winter but hasn’t effected the mileage as much as those three random times. As noted. Getting 46.7mpg right now in temps of 30-40 degrees. Something else was going on at those times. Who knows. Fortunately not persistent

 

After reading this again I’ll offer another theory. While you were in HV, the ICE may have been working to restore the SOC set point. I’ve observed this even on downhill sections. So, possibly, regen was limited because the ICE was charging.

Thanks for the clarification.

 

Additional theories...

It is winter folks. I heat my house with natural gas; however, my electric bill goes up in the winter due to increased electric usage from: HVAC running more, electric blankets, electric throws, additional light usage (it's dark more than it is light,) lower humidity levels make it "feel" colder therefore wanting it to be warmer (additional heat, additional HVAC fan usage,) being inside and using more other things for entertainment - TV, electronic games, etc. (instead of being outside more.) I don't believe the increase in electric cost has much to do with the Clarity, but rather it being Winter!

With my Clarity, when I am stopped in cold weather, with the ICE off, the heater keeps on running. They said that they only use gas. With the Clarity in a Zero Charge condition, it is going to use more gas to attempt to charge the battery so that it can be used in HV mode - meaning that it is going to attempt to charge the battery about 10% - whether it actually is reflected in the GOM or not. When the battery has some charge in it, and ICE is off, then it is going to pull as much from the battery as possible to generate heat - like when first you do the first drive, whether you do remote climate or not. Heating our cars in cold weather pulls a lot of juice from the battery.

Personally, I have a 3-car garage with a 12-foot ceiling. My Clarity occupies the single-car slot, and the double-car slot is used by my wife. If I want to work on the Clarity (rotate tires, oil change, clean it out, etc., I move my Clarity to the larger side where I have more room; but, with the EVSE disconnected, I also will roll down the windows and put the HVAC on HIGH. I can warm my garage easily this way. Even with temps in the 20s, after about 1/2 hour, it is comfortable in there. BUT - it uses EV battery. I can get about an hour of heat out of the Clarity before I have to recharge it. Based on this, and my casual scientific and mathematical abilities, I conclude that the Clarity can use approximately 1KW of battery every 4-5 minutes when the heater is trying to warm the car in the COLD. I also use my Clarity as a garage heater when I need to do maintenance on my wife's car - or I get bored (thank you Covid!) and want to clean out areas of my garage.

 

I’m not clear on how the DC/DC converter can produce the 14,000 watts that would be necessary to completely drain the HV battery in one hour. If you’ve done it, I’ll take you on your word. 14kW converts to ~48,000 btus.

An alternate heat source may be something to consider. A couple of 1500w electric heaters would only use 3000 watts per hour. And the heat could be directed at the work area.

 

At 60mph in pure EV mode, the Clarity will drain a fully charged battery in 47 minutes, assuming the Clarity gets the specified 47 miles per charge. That comes out to an average of just under 18KW power for those 47 minutes. For those who like to think in muscle car terms, that is an average of just under 24 horsepower for those 47 minutes.

In energy used terms David is correct, 14 kilowatt hours is equivalent to just under 48,000 BTU's. Two 1500 watt electric heaters put out just over 10,000 BTU per hour so the garage would be much warmer using the Clarity as the heater. Actually, the heating efficiency (BTU per KWH used) is the same for the Clarity and the heaters.

LeoP

 

I understand how the traction motor can consume 14kW’s in one hour. Can you explain how the heating system can do the same?
Is the resistance heat operating on 12V? What amperage fuse is protecting the circuit for the heater element?
Is there any chance that a 48,000btu furnace might melt the car? I have a 13K btu propane wall heater that warms up a 15x30 room from 42F to 68F in less than 30 minutes.

One owner in our motorhome group has installed a 10kWh lithium battery. He can run a 13.5K btu A/C, residential refrigerator, TV, microwave, etc for hours. He’s going to install a mini-split. Help me understand how the Clarity can consume such a tremendous amount of energy to heat the interior.

 

My two cents: A couple of weeks ago I was trying to drive back from Vegas to LA but the 1-15 mountain pass was closed due to snowing so google maps rerouted be southwards through searchlight and then through the desert. I was diving in HV mode and I soon noticed the mpg went dramatically down, I mean dramatically as bad as 18 mpg! I slowed down to 60 mph but the mileage stayed below 25, so I was afraid something had gone wrong with the car or I might have a leak so I wouldn't be able to reach the next pump. When I finally made out of the desert and arrived to the gas station with 15 miles remaining range I finally realized there was a horrible head wind I was having on the road, which barely allowed me to open the door and get out of the car to pump gas. When I was back in LA the mpg went straight back to the middle 40s driving at 70 mph. So driving the Clarity against a 75+ mph head wind will dramatically reduce the gas mileage. Add this one to the multivariate model explaining the somewhat bizarre behavior of our beloved car...

 

The wind load quadruples as the speed doubles. At 75mph with no wind it takes 25-30hp to overcome aerodynamic resistance. If it truly was a 75mph headwind the car would need to put out 100-120hp for aerodynamic resistance alone.

 

I'm pretty sure the heater runs off the high voltage side (as does the AC compressor, with only the fan running on 12V). From what I've read the electric heaters in EVs are in the 4000-5000W range, which would indeed be too much for the DC-DC converter -- and would require VERY thick wires to handle 300+ Amps at 12V. If it were a simple 12V fuse I would have pulled it or added a switch, because I don't want the heat coming on most of the time

Car heaters and ACs are a lot more powerful than you'd expect, because of the need to heat and cool quickly and the huge portion of glass. A typical car air conditioner might be 5 tons, which is the size of a central AC in a pretty large house.

 

I found a part number for the Electronic Coolant Heater, 79900-TRW-A01. It’s about a $2000 part. The diagram shows a relatively small snap connector that attaches to the harness. I couldn’t find the operating voltage or wattage, but would appreciate knowing if anyone has that info.

Even at 4-5Kw it would take ~3 hours at maximum output to drain the 17kWh battery.

 

To the OP, someone (KentuckyKen) on this forum helped me a long while back with some unusual issues I was seeing. They were not the same as described here, but the remedy is unlikely to hurt anything. Ken had me disconnect one the battery leads (likely I'd do the negative). Wait a few seconds and re-attach. The car will complain with all manner of scary warnings after re-attach. But drive the car about 10-15 miles and everything will go back to normal.

It's sort of a way to reboot the car. It's very unlikely to hurt your car (as we'll all have to replace the 12v battery eventually), and it may help if something is screwy with the cars settings.

The one thing my car consistently did was running in HV mode the EV range would tick down as I drove. This went on for many months and I didn't know it was abnormal. After the reboot, pressing HV keeps relatively the same EV range even over a long trip. It will drop a few miles, but it won't keep dropping to zero.

 

Yay, Kentucky Ken! Boo, InsideEVs--they've removed @KentuckyKen from their pop-up list.