Understanding Prime Reported MPG With EV Mode Capacity

I have completed the test and believe the car's reported MPG in HV mode is totally bogus when battery SOC is anything other than ----% for the entire HV mode miles.

My test:
- 248 miles in
- 34 "HV Mode" segments over 10 days,
- always selecting HV mode at startup
- eco driving mode,
- AC on 75F, outside 90-95F,
- Florida flat,
- Suburban 1-3 miles at 45mph between lights,
- starts with slight power region usage until close to speed limit
- 2023 Prius Prime XSE
- tires at 35
- Filled when half tank used

- Fillup: 5.091 gallons (at $3.889)
- Delta SOC: -60% (100% ... 40% at fillup)

Results:
- The average of the 34 reported MPG at shutdown: 52 MPG
- My calculated "HV Mode w/available EV mode charge" average: 43 MPG
- $0.090 per mile cost for HV mode miles at $3.889 per gallon gas ($0.045 cost for EV mode miles)


Methodology:
- "HV miles" = Total Miles - (Delta SOC * Average EV Range)
= 148 - (.6 * 45.8) = 221 miles
- 221 / 5.091 = 43 MPG

Evaluation of Reasonableness:
- The average MPG for the first three fillups (HV mode ----% for all miles) of hours long highway miles segments was 45 MPG.
- Only 10 of the 247 test miles were at highway speeds, with the remaining segments averaging 7 miles of stop and go up to 48 mph, so 43 MPG seems reasonable.

Comments:
- So glad this is done and I can return to EV mode driving -

 

Yes, the purpose of this test being lots of short segments in HV mode was to compute cost savings of EV mode vs HV mode for my everyday around town trips.

I won't get opportunity to test long HV segments with avail SOC until I can solve the new bike-rack need or a future vacay.

 

While I am not trying for hypermiler status, I do try to limit the time in "power" on the eco meter as I drive, and obsessively record the end of the day SOC and total EV rate (mi/kWh) and the next morning record the charge from the wall (kWh and $) so I know how much to transfer to my utilities budget category, and to satisfy my desire to know/understand everything in my life (gave up understanding people)

 

I don't use Power Mode. I'm leaving it in Eco mode, with Eco AC (since I don't have to worry about ice on the windows ).

The "power" I was referring to is in the Eco arc above the vehicle speed. There are two green sections labeled "ECO", and two white sections labeled "PWR".



I have found that starting up from a stop light with cars behind me, I need to accelerate with just a small sliver into the first PWR section. If no one is behind me, I will usually keep it just below the power section and take more time getting up to 45 mph.

Today I went back to driving in EV mode. My average EV mode consumption rate is 4.3 mi/kWh, but as the above shows today I was in a bit more heavy footed, (and also needed serious windshield wipers).

31.6 miles at 4.0 mi/kWh means I used 7.9 kWh of the available 10.6 kWh for EV mode. As a second method of calculating how much battery I used - the SOC reads 24% left so 76% of 10.6 kWh is 8.0 kWh so pretty close. My charger will pull from the wall 1.3 times the battery used, so tonight's charge will be roughly 10.4 kWh, cost $1.49, and take 7.5 hours. $1.49 for 31.6 miles at today's $3.889 per gallon is the equivalent of 82 MPG! Even being a little lead-footed I'm still feeling good about that.

 

If I sum up the (miles times MPG) for my test I get an estimated 4.7 gallons of gas used for the 248 combined miles giving an average near 53 MPG.

The actual fill was 5.091 gallons which again ignoring the grid energy used would give 49 MPG which is 8% lower. Not good but wait…

If I don’t ignore the roughly 27 “grid miles”, then the actual consumption rate is approximately 43 MPG which is 23% lower than the average indicated making the displayed MPG in the Prime totally bogus.

The prime appears to be able to measure battery power consumption extremely accurate ( +/- 1% ) but gas consumption only to -20%.

 

I appreciate your participation, but hasten to suggest checking the topic of the thread.

In this thread, I am hoping to understand the multiple ways the car reports HV mode Miles Per Gallon and find something in the Prime that approaches my 2009 Prius MPG indication which was never more than 4% higher than the average of the actual MPG.

Because of the bladder tank, the car's MPG indication could appear to be wildly different, more and less, than the (miles-since-last-fillup / gallons-this-fillup), but on averaging these the indicated remained around 4% higher than the averaged actuals.

 

Indeed, I expected to find the indicated MPG to be around 5% high similar to my 2009 Prius which was 4% high on average.

Discovering the optimism to be 23% is frustrating, but perhaps just as single actuals were sometimes wildly different than the car's reported MPG in the 2009, I would need to perform this HV driving test multiple times and average the actuals. I like driving in EV mode too much to invest further in HV mode MPG testing. I was hoping the result would be closer.

 

Yes, I want the car to estimate the real, actual MPG not some "ignore reality" value. It is very convenient for the Prime Prius to use the non-Prime Prius MPG estimate, but it loses its value in the Prime (when there is EV charge available).

 

Some folks have reported that it does hold the SOC approximately on longer drives at highway speeds (manufacturer suggested optimization). I seem to be confusing the car with shorter drives in a suburban setting.

Bottom line, I was trying to do a quick and dirty test to compare HV around town with EV around town. The car says, "no I'm still going to use EV because it is more efficient", and "the devs did not invest in writing a calculation for a non-usual use case."

 

Came to that realization the hard way.