What is your DTE after fill-up?

I don't think comparing the DTE with actual miles driven from a full tank on regular Gen4 Prius or PP on HV only is the way to find out the accuracy of the HV only mpg based on the DTE. Besides, the DTE is an estimate for driving distance until the empty light comes on. If you fill the tank before the light comes on or drive much further past that point (on PP with a full charge, you can theoretically drive ~130miles beyond the empty light when DTE reaches - - mile), you will never be able to compare the DTE to the actual miles driven. The actual miles driven is affected by the current and future driving conditions, and DTE is based on past performance. Similar to the GOM EV range, the DTE is only a prediction at the best. But what I think DTE is good for is to use the number to estimate "past" efficiency.

Here is some data I have on using DTE to estimate HV only mpg on my last PP. The number difference between the calculated (full tank method) and DTE estimated is only +3.6% for the one full tank drive I did without charging (no EV drive, 4th row) and -4.8% on the one drive with a single charge (comparing to adjusted mpg, 5th row). For adjusting HV only mpg, I used 5.0miles/kWh for the EV efficiency. I used kWh from the wall for calculating EV miles driven, but this is actually not accurate for some portion of the kWh used for charging from the wall never really goes to the wheels. So more EV drive I have done during the full tank drive bigger the % difference when comparing the adjusted calculated HV mpg to DTE estimated HV mpg (rows 1-3).

 

Humm, here goes a new data collection project for me. Nop, I have not recorded pre-fill DTE. I just try to fill the tank as close to empty light, but I admit that I have not always done so.

 

Thanks for the n=1 data on this. Just for the completeness of the data set. Did you happen to set the trip meter to zero before this tank and checked the corresponding mpg display from the dash at the time you filled it recently? Of course, that number contains ~20mils of EV, but even adjusting for that extra miles, I would think it is even more inflated than the DTE method. Historically, on my previous PPs displayed mpg was 6-7% inflated from the calculated which was also true for my Gen3. I have no doubt our 2021 PP is the same in this aspect.

Unfortunately, for my 2021 PP, I am currently running it as much on EV as possible to save money. Until very recently, the gas price was cheaper than electricity so that, I was running more HV than EV. That made me refuel more often to get more data points, but now with my driving habits, the refuel interval is likely to be more than 2 mo.

 

This is the entire driving record of my 2020 PP. The % delta for the mpg displayed vs mpg calculated (this is just calculating distance driven/gal of gas at fill-up without adjusting for EV miles) was average +6.82% inflated on the dash display. But if I do not include the very first top-off fill of 1.7gal after purchasing the car which gave a very accurate but very low mpg with only -0.01% delta, the average would jump to +7.96% inflated mpg display on the dash.

That might be the hardest part of your experiment.
Easier way would be to not charge the car at all after the fill-up. Of course, you also have to have the EV range depleted before the fill-up.

 

The "9.3" factor came to play because I assumed the empty light first came on at that point leaving ~2 gals of gas still in the tank. This assumption may be wrong. The empty light may come on at a somewhat variable rate. I think I have had the empty light came on and was able to stop for the refill immediately after that and the car took ~10 gals of gas, meaning it was more like 1.3 - 1.4 gals (depending on which spec you are looking at) still left in the tank at that point. So, that would make "10" the magic number.

From @road2cycle data, if Odometer calculation is the most accurate mpg, then adjusting the factor accordingly would give us a more accurate HV only mpg estimation. 528/53.49=9.871... So, using 9.87 for DTE estimation would have given the accurate mpg. If we have enough samples, we can adjust this number to a more accurate one, but the number is likely to be variable anywhere from 9.0-10.0. Using "9" would give an inflated more optimistic mpg, whereas using "10" would give a conservative more pessimistic mpg value. Maybe we may settle for 9.5 as the best number to use?

 

Wow, that's impressive. If the 9.3 factor is correct that would be ~82mpg on strictly HV mode.

Just curious, what is on your ODO and your lifetime MPG on ODO? Of course, the mpg displayed is inflated by EV mode, but it seems you do a substantial amount of HV mode drive.

 

WOW! That's amazing. Is your regular driving distance always substantially longer than the EV range?

I use to drive on HV more often even for the shorter distance because gas was cheaper, but now the gas price is higher than electricity. I am doing most of the drive on EV. That leaves only very few occasions, often on high-speed highway drives for HV mode. I have to burn some gas I have in the can for my generator so I can refresh the stock for the winter. I should be refilling the tank soon for the third time in ~3500 miles.

 

Is that because your electricity rate is exorbitantly high? Even with your hypermiling 85mpg or above efficiency on HV mode, from what I have heard on the price of the gas in CA, I think if your trip is routinely shorter than say 40miles total per trip, driving on EV mode most of the time would be cheaper, wouldn't it? Our gas price is currently at $3.35/gal. At this price and compared to unchanged $0.21/kWh electricity rate, even if I get an average of 85mpg on HV mode, EV mode with 35miles of the range is cheaper. HV is $0.054/mile vs. EV is $0.039/mile.

BTW, this number was completely opposite not too long ago, when the gas was near $2/gal. No matter how much I could have extended EV range, the HV was cheaper.