Highway mileage lower than city mileage

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kcsunshine

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Hi, I noticed that my mileage is better in slow traffic compared to high speed driving. This is with a full charge. I have a round trip commute that is at the limits of EV range so I noticed that if I'm driving at highway speeds, the ICE needs to assist to get me to my destination. Someone said to me that this is due to the regenerative braking but I don't feel that is correct. I understand that regen braking will recapture some of the energy used to propel the car but you can't recapture all the energy used or gain energy with regen. If you could recapture all the energy, you would be driving like a crazy person and race forward and stop. I did a search and autotrader claims it is regen braking. So what is the real reason slow speed mileage is better than high speed mileage? I say it is because of high speed drag. The air acts as a greater force at higher speeds. I'm not comparing a hybrid car to a regular ICE car.
 
Tesla says their cars are most efficient when driven at 23 mph. Obviously, if driven even more slowly there would be less drag so there's something else going on as well. ICE cars are most efficient when driven at around 50 mph, generally the slowest speed for the top gear. After that, wind resistance would counteract any gains by gearing changes. You are correct on regen braking not recovering all the energy. If you really want to hypermile your EV, you'd want to find a flat road or track and drive at a very low constant speed. I watched a video where some enthusiast did that with an Ionic EV and he got more than double the stated range by going super slow. It was taking so long to deplete the battery that he sped up at the end just to quicken it. For EVs and hybrids, speed is your enemy.
 
All vehicles are more efficient at lower speeds due to drag increasing exponentially at higher speeds. The reason traditional ICE cars get lower gas mileage in the city than on the highway is because of the constant braking and acceleration, which uses up a lot of extra energy. Regenerative braking counters this, recovering a lot of the wasted energy from when you brake. This is why hybrid and electric cars actually get better gas mileage in the city. All traditional hybrids like the Prius, Insight, etc, as far as I'm aware, advertise higher MPG in the city than on the highway. The Clarity's electric range of 47 miles is just an estimated average. I've only been driving it for three weeks, but in my experience, it's probably around 45-47 at interstate speeds and 50-53 or so at lower speeds.
 
Regen gives you better mileage/range than you would get braking similarly without it, but the primary reason for better city mileage is just the lower speed as mentioned above. In fact, one of the best ways to extend your range is to avoid letting the car go into any default regen and only brake when you need to. Since the car doesn't coast when you let off the accelerator, this takes some practice. In our area of country roads with limits 30-40 mph, just avoiding unnecessary acceleration and unnecessary braking will take the EV range estimate into the upper 60's without trying very hard.
 
As you know at low speeds the ice will not come on as often as it does at high speeds because of drag. Other factors at high speed include friction from your tires, increased resistance from your drivetrain,, the grade of the road, and also a coarser road surface will increase resistance. Something else I complained about is the adaptive cruise control. It will slow the car abruptly as you approach another slower moving vehicle then rapidly accelerate to the set speed when clear.This is not very efficient and also pulls down the mileage. They need to reengineer the acc if the goal is efficiency.
 
This is normal for hybrid cars, city mileage is almost always higher in the city. On this car, as most EV cars the single speed gear for the EV motor is optimal for low speed and high torque. A paper from Honda had a graph showing the optimal speed for the EV motor was in the 28 mph area. That's why this car also has a ICE gear which is like "6th" gear, basically the highway cruising gear. It it optimal at highway speed.
 
I've achieved 85 mpg for a 10.6 gallon tank of gas in my 2006 Insight by keeping my speed below 40 mph. However, by staying below 20 mph, some crazy people achieved nearly 115 mpg for a tankful, going more than 1500 miles. Honda's i-MMD hybrids always achieve higher EPA urban ratings than highway ratings, but the old IMA hybrids, such as the 1st two Insights, did better on the highway than in the city.
 
I believe this graph explains why. For any given car aerodynamic design, there is much lower vehicle drag force & associated energy loss at low speeds. For gas cars, low speeds stop & go, they lose kinetic energy from braking and idling. For electric, we can recoup kinetic energy via regnerative braking and avoid engine idling losses. Look at the Blazer curve for example. The drag force is over 400N @ 60mph vs. 100N at 30mph, 4x difference. If all things kept constant, except for velocity, then high speed driving is less efficient.
drag_plt.png
 
Remember, with no gears to shift through, the electric motor rpm increases arithmetically or linearly while the aerodynamic resistance increases geometrically or by the square of the speed. This is depicted in the above graph with drag vs speed being an exponentially shaped curve.

This coupled with much more regen (since slower speeds usually also includes disproportionately more opportunities for regen than faster highway driving) explains why the EPA city cycle number is higher than the highway number for EV driving.
 
neal adkins said:
As you know at low speeds the ice will not come on as often as it does at high speeds because of drag. Other factors at high speed include friction from your tires, increased resistance from your drivetrain,, the grade of the road, and also a coarser road surface will increase resistance. Something else I complained about is the adaptive cruise control. It will slow the car abruptly as you approach another slower moving vehicle then rapidly accelerate to the set speed when clear.This is not very efficient and also pulls down the mileage. They need to reengineer the acc if the goal is efficiency.
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Neal why does your ICE come on at all? As long as we have Econ on and drive not to aggressively our ICE will only come on when the battery is depleted. This is why we can drive for weeks without the engine ever coming on. I don't really understand your comment about, "the ice will not come on as often"
 
insightman said:
... Honda's i-MMD hybrids always achieve higher EPA urban ratings than highway ratings, but the old IMA hybrids, such as the 1st two Insights, did better on the highway than in the city.
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Having owned Honda Civic IMA Hybrids (2003, 2009, & 2010), I can attest that the mileage was better in city than on highway. The unrecoverable losses due to wind drag always made sense to me.
 
jdonalds said:
Neal why does your ICE come on at all? As long as we have Econ on and drive not to aggressively our ICE will only come on when the battery is depleted. This is why we can drive for weeks without the engine ever coming on. I don't really understand your comment about, "the ice will not come on as often"
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when on the freeway I sometimes drive 120 miles or more round trip. So I use hv mode and save ev mode for low speed driving. On my longer route,about 85 miles from home, i climb a mountain almost 1mile high, so I use hv charge mode to have maximum power. So i only use hv when nessacary.
 
ClarityDoc said:
Having owned Honda Civic IMA Hybrids (2003, 2009, & 2010), I can attest that the mileage was better in city than on highway. The unrecoverable losses due to wind drag always made sense to me.
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EPA 2003 Civic Hybrid: 39 city, 43 highway; EPA 2009 Civic Hybrid: 40 city, 45 highway; EPA 2010 Civic Hybrid: 40 city, 45 highway. Of course, as the EPA always warns, YMMV.

In my 2006 Insight I averaged 64 highway mpg through 8 full winters and summers. Then I stopped commuting and began mostly city driving. Now I'm lucky to get 55 mpg on a tankful of gas and my lifetime average has plummeted to 59 mpg. The precipitable drop in fuel economy took all the fun out of trying to be a hypermiler. Having never taken our Clarity to the gas station since we purchased it in December has made my Insight feel like a gas guzzler! Now I'm hoping Honda will bring their ultra-cute 2020 Urban EV to the US, instead of just Europe. Then I'll have to change my handle to the unwieldy URBANEVMAN.

upload_2018-9-15_10-59-55.webp
 
I hope that Honda will retain the majority of the design element in the production car. This car is such a chick magnet :) I like the design outside a lot, especially the unique headlights, but inside is not practical for daily use car.
 
One of the ways that you can tell that "drag" is the real issue here is to simply look at the bizarre designs of all of these electric cars. They're all designed to minimize wind or road resistance in multiple ways. On the other hand, even though I spend 90% of my driving time on freeways and at fairly high speeds, I am not bothered in the least by the decline of efficiency. I find that the whole issue of the ICE being on or off is rather irrelevant on the freeway. Unless I look at my panel I often can't tell if I'm on pure electric or if the ICE has kicked in. So yes, my mileage suffers because it's all freeway but the upside is that I don't care too much about eking out every electron because the ride is quiet and comfortable even on HV.
 
I went on a roughly 440 mile road trip this weekend and achieved a whopping 37.8 mpg according to the trip computer, Granted it was cold going up the mountains to a ski resort, but yeah I left the house with a full charge and came back home with 3.8 miles of EV range left even though I was on HV mode the entire time, I had to even use HV charge for a bit because the battery was going just draining at a torrential pace.
But yeah 2 tanks of gas, wasn't really efficient this weekend.
 
Wdave said:
I went on a roughly 440 mile road trip this weekend and achieved a whopping 37.8 mpg according to the trip computer, Granted it was cold going up the mountains to a ski resort, but yeah I left the house with a full charge and came back home with 3.8 miles of EV range left even though I was on HV mode the entire time, I had to even use HV charge for a bit because the battery was going just draining at a torrential pace.
But yeah 2 tanks of gas, wasn't really efficient this weekend.
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And in truth if you actually calculated your mileage instead of just reading Honda’s exaggerometer, you would find you actually got 34 or 35 mpg.

Cold weather and uphill? If that’s really quite good for most any hybrid...or other car for that matter.
 
craze1cars said:
And in truth if you actually calculated your mileage instead of just reading Honda’s exaggerometer, you would find you actually got 34 or 35 mpg.

Cold weather and uphill? If that’s really quite good for most any hybrid...or other car for that matter.
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It's probably worse I had the car warming up in HV charge before heading into the car so it was nice and toasty .
Too bad this car soley uses a resistance heater for the car.
 
It doesn’t. It uses engine heat too, if the engine is running. HV charge is known to reduce mileage because it raises rpms higher than would otherwise be needed to simply maintain a charge instead of add charge. I never have engaged it for that reason....including on a 4200 mile mountainous road trip. I see no practical use for that feature, it’s wasteful. If battery drains it drains, That’s ok. Let the car just decide what it needs to do...yes the engine will scream at times....and yet it can be surprisingly efficient.
 
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