Questions re: highway mileage on flat ground & charging logic

Thanks for providing a good amount of info on your first post and mileage query! :thumb:

Was your rental a touring? Per Most fuel-efficient cars, the touring incurs a slight mileage hit vs. non-touring. What's the make, model and size of tires on your car now?

What are the outside temps of your recent run vs. when you had a rental? Close your windows. Having them opening will surely increase drag.

Out of curiosity, what mileage do you expect and why? I gather 49 mpg is part of it from the rental... Does part of it come from EPA estimates?

 

When it was green and still charging were you going more than 52mph and not let off the accelerator fully since before you went that speed?

It should be rare to get regen 50kwh car icons on flat terrain. It is possible, but terribly inefficient.

When you did accelerate harder, the battery did drain some correct? And when you say it went down to 44mpg, over what range is this? Are you computing mpg over each of your trips, or are these tank averages over a few months of various driving methods?

This seems low to me. A 12v battery should not read below or even at 12v when in good health.

The maintaining of green charge is suspicious. It is hard to tell if you should see more assist or not. There are times I make it up my mountain (6% to 9% grade over a few miles of curvy 25mph road) with the same charge I started at, but usually I am at 2 or 3 bars shortly after I start and maintains that to the top. When I go up I-70 which is steep interstate, any of the hills at 80mph-85mph deplete the battery quickly.

Well you posted lots of good information, to gleam info from. It is refreshing after reading most of the posts around here. As mentioned, the touring edition with its larger tires will get you a hit. Not 10%, but a few mpgs.

 

I drive on level ground with a 2009 non touring.

At 61 MPH I get 50 MPG on warm days 80F
At 68 MPH I get 45 MPG on those same days.

 

One other thing to keep in mind - if you're using heat or defrost, it will keep the engine running more than normal, and definitely affects my mileage on a short commute. Although if it's warm enough now that you can crack the windows open, you probably aren't using it too much. I don't think the windows would affect your mileage too much, unless it's more than a couple inches and you're going at a higher speed.

In the summer I generally drive with the driver's window down about 2-3" and the right passenger's window down about 2", that gets a good airflow all thru the car, without opening up a lot of drag. If the back windows are open more than the front, you get a disturbing 'beat' noise.

 

First, I am not sure if I understood correctly, but how long was your highway-only drive? You really need to spend something like 200 miles on the highway to reach a conclusion on your mpg.

First, at 65-67 mph you are not keeping your speed down. The Prius is not very efficient above that speed. You will not be able to get good mileage if 65 is your floor speed because you cannot glide (no gasoline injected) from 67 to 65 mph. My experience is that you have to declare the floor speed to be 60 mph and accelerate efficiently until 68 mph, then glide again down to 60 mph.

44 highway mpg is not alarmingly low, but it's definitely on the low side. You could have a problem, but unless you optimize your driving it would be impossible to determine and expensive for a dealer to check.

I don't know if you have a scangauge, but it is a necessary tool if you want to monitor RPM, kW, BTA/Voltage or injection timings. (Amperes/Volts). It is very hard to go above 50 mpg without these on the highway. If you do try and keep your RPM between 1650 and 1850 when accelerating or ~15 kW.

Now in terms of the 12 V battery, a one-time measurement will not give you an answer about its condition. Do the following:

1) Determine how old the battery is.
2) 11.9 V under load is not bad. Mine was 11.6 V under load (and 3.5 years old) before I changed it Take it for a long (6 h) drive or leave the car running o/n to force the battery to be charged. Turn off the car and allow the surface voltage to fall for two hours then check it again. Battery health is more determined by whetehr the battery can keep its charge more than finding it discharged. Think of any battery you have ever used, when they go bad they cannot hold charge. When they go low we charge them back. If after your attempt to charge the 12 V battery the charge remains at 11.9 V then your battery may be faulty. Though I don't think that that could cause low highway mileage. A more common symptom is the inability to lock all doors when you press the driver's, passenger's or the trunk's buttons.

3) Regarding the high state of charge of the HV battery. That is the most worrying sign if true. Again a scangauge II with battery Amps/V and SOC programmed would answer your questions. BTA shows positive or negative values depending on whether you're charging or discharging from the HV battery. The S.O.C. % is a more accurate indicator than the green and blue bars.

 

I think the next test is to get a good baseline. Find a relatively flat stretch of freeway or highway that has no stops and little traffic. Make sure you are in S4 before starting. Reset your trip computer as soon as you get on the interstate. Drive 55mph, windows up, AC/Heat off for a good length of time, something in the neighbourhood of 20miles or more. Then turn around and drive the exact same route back, same speed, same settings. If done with relatively similar environmental conditions (the full trip was sunny, not that it hailed on the way down and was sunny on the way up), the terrain differences will average out, and you should get a good reading of what the actual mpgs are.

The smaller the distance you go the less accurate obviously. Extrapolation is a dirty beast.

I still think your mpgs are a tad low for your driving. Unless you aren't being forthcoming with your speed. Many say under 70mph because that's their local speed limit, but they really go 73-75mph. Which will make a huge difference.

But as a comparison, my non-touring GenII gets 44mpg average, when driving a 15mile stretch of interstate at 80-90mph nominal speed with bursts of 100-105mph through the gulleys where it is hazardous for any cop to hide in.

Driving like grandma in the slow lane at 55mph nets me 50-52mpg for the same stretch.

Driving like a true hypermiller around town, I can get it up to 60mpg but not more since my commute home consists of those few miles up steep grade at single digit mpg's which just destroys and hard work throughout the day.