Hi folks,

I plotted the MPG versus temperature for three NHW11s that each had over 100 samples in the database:




The first one is my 2003 Prius and the others were taken from the database. It looks like I do a fairly good job at all temperatures except 70F/21C. It may be because I slack off on my warm-up / cool-down cycle when it is 70F. Otherwise, I don't know what is going at 70F but plan to find out.

Note, I just started using an engine block heater this year after installing it this summer. Last winter, I tested my thermistor hack and that seemed fairly effective.

Bob Wilson

 

This is a good info. I have noticed the weather condition impacts Woody's ( OK, it's my 2007 Prius' name. ) MPG performance.
When temperature drops to around 70 - 80 F, he has better performance compare with the days in the 100s.
When it rains, now this is a dangerous and tricky part I have found, if it's light rain, he has the best performance ever. Why, because there is less friction on the road....

 

Bob,

My analysis is less detailed than yours, but my MPG records per month show the variation you describe. Of course temperature is only one of the variables (wind and precipitation also vary by month). See the attached spreadsheet for details (almost 4 years and 100,000 miles - no more warranty ).

JeffD

 

This is all good data! I got my 07 in September and have been tracking it by the MPG on the MFD, MFG calculated the old fashioned way, and cost per mile.

I'm still aggravated by the two MPG numbers. The MFD number has stayed fairly consistent but the manual calculation swings all over the place. I know that part of it is the flexible bladder in the tank but can the number on the MFD be trusted??

 

Have you also considered the hybrid battery's "flexible bladder"(***)...?


*** Variation in charge state at fill-up.

 

I once did a calculation of how much gasoline the variability in battery SOC amounted to - IIRC, it was about a half a pint. IOW, less than 1 percent of a 9 gal fillup.

 

The number on the MFD is usually more accurate than calculating one tank fill. If you average your fill-ups you can get better accuracy than the MFD over a two or three tank average. But with temps dropping right now, you will not get very far this way. As temps drop, MPG drops. Main cause appears to be fuel lost "warming up" things, such as the car interior, the cat., the engine, and the oil in the transmission. Once outside temps stabilize you can get a pretty good estimation of fuel economy over several tanks.

For reference, I'm filling up about once a month right now. August and September's fill-ups averaged at 4.2 l/100km (56 MPG US) - MFD claimed 4.4 and 4.9 l/100 km. Just filled up today, and got 5.3 l/100km (44.4 MPG US). MFD claimed 5.1 l/100 km. Yes, it's colder (avg just over freezing at 3-5 deg C). I try to be consistent when filling, but I sometimes get greedy, and try to fill as full as possible without seeing fuel in the filler neck. So some of the variation is due to that.

 

Thanks for all the input. The battery has a 'flexible bladder' too? I wasn't aware of that. I'm filling weekly (more out of habit than anything and trying to make sure I don't get caught with an empty tank and gas prices through the roof).

If I take all tanks that I've filled since I got the car 2 months ago, then the avg is pretty close to the MFD.

 

Bob,

Have you noticed any mileage gains since installing the EBH?

 

I think what WWest means is that when you fill up with gas one time, your battery will be near empty, resulting in slightly lower mpg on that tank. Or it may be full, giving you a bit of a head start.
Personally, I don't think it will make a noticeable difference, because the battery is constantly charging and draining.
Like memiller said, the difference is less than 1% of a tank, which to me is negligible.