That's a lot of loss at highway speed (with a twist)

[Rocky_H]

I just went on a trip and noticed a big drop in efficiency at highway speed. I was going at 80 mph and lost over 29% versus what I usually get driving around town.

OK, now comes the twist: this wasn't my electric car. I see threads all the time with new owners complaining about Tesla's "false numbers" because they drove fast and weren't getting the rated range, and frequently there are claims that their gas cars never had that problem. Well, this WAS my gas car.

We were taking our 2005 Honda Civic Hybrid, and I got on the interstate in Idaho and drove at 80 mph for two hours and only got 31 mpg versus the 44 mpg we always get week in and week out in our normal driving around town. So yeah, a 29% drop in efficiency because of the high speed.

I just wanted to have a bit of an eye opening counterpoint to all the griping I see against electric cars to point out that physics is a cruel mistress that affects all and is a witch with a capital B at 80 mph!

 

[beatle]

At least you didn't have to turn on the heat! Similarly, people attribute poor gas mileage in the winter to the engine running richer at startup to bring the engine up to temperature - and this is true, but even on a long trip, mileage is worse. It's also true that an engine needs more fuel when getting cooler air to maintain the same air/fuel ratio, and that means more power (this is why cold air intakes are popular) but when cruising you only need as much power to overcome rolling and wind resistance to maintain your speed, so you'd just draw in less air to make the same power... unless you need more power to overcome more wind resistance in colder air (hint: you do).

Most are familiar with losing range in the winter, but it's often blamed on the fact that the cabin and battery heating elements consume power. This is true, but when I tested this last winter by driving 2 hours with a pre-warmed battery in cold temperatures and I only used the seat heater, I found my range didn't really improve much. In reality, air temperature makes a good dent on consumption for both ICE and electric vehicles. I used these two pages to calculate the wind load on a Model S at various speeds and temperatures. I set the drag are of the S to 0.575999 square meters and atmospheric pressure to 1 bar. Note: this is amateur hour since I am not an engineer, so maybe someone can check my math.

Wind Load vs. Wind Speed

Wind load on surface - Wind load calculator.

www.engineeringtoolbox.com

Air Density Calculator

Use the online air density calculator to find out the density of air at any given temperature and pressure.

www.omnicalculator.com

Temperature	Air Density	Wind Speed (m/s)	Speed (mph)	Wind load (Newtons)
32F (dry)	1.27537​	35​	~78	450​
80F 60%RH	1.15273​	35​	~78	407​
32F (dry)	1.27537​	27​	~60	268​
80F 60%RH	1.15273​	27​	~60	242​

The load is almost 11% higher in colder temperatures. Load is also a lot lower at lower speeds as Rocky_H said.

 

[slainla]

I just went on a trip and noticed a big drop in efficiency at highway speed. I was going at 80 mph and lost over 29% versus what I usually get driving around town.

OK, now comes the twist: this wasn't my electric car. I see threads all the time with new owners complaining about Tesla's "false numbers" because they drove fast and weren't getting the rated range, and frequently there are claims that their gas cars never had that problem. Well, this WAS my gas car.

We were taking our 2005 Honda Civic Hybrid, and I got on the interstate in Idaho and drove at 80 mph for two hours and only got 31 mpg versus the 44 mpg we always get week in and week out in our normal driving around town. So yeah, a 29% drop in efficiency because of the high speed.

I just wanted to have a bit of an eye opening counterpoint to all the griping I see against electric cars to point out that physics is a cruel mistress that affects all and is a witch with a capital B at 80 mph!

You actually made a case against your point. You have a hybrid so it's not a just an ordinary gas car. All hybrids, and BEVs, have less MPG or range while driving on the highway due to the lack of braking/stop and go traffic. The reason for this is because of the way EVs are designed.

All normal gas cars will generally have much higher MPG while driving on the highway, even at 80mph, compared to driving around town.