Another Road Trip Report

[Reyn]

Earlier this summer, my wife and I did a week-long road trip in our 21 Clarity PHEV from southern California to various parks in Utah and back, and we got a gas-only mileage of 48.2 mpg (see thread titled Road Trip Report.) This past Thanksgiving weekend we drove our 21 Clarity PHEV from southern California to Cupertino (in the San Francisco Bay Area) and back.

TLDR: estimated gas-only mileage 50 mpg

We started with a fully charged battery and full gas tank, and except for the first few miles, we drove in HV mode exclusively. The first few miles were driven in EV mode to drain the battery slightly so that it would not be at 100% for long. The battery was not externally charged during the trip. At the end of the trip the battery SOC was 72% (OBDII reader using Car Scanner). We used a total of 14.464 gallons of 87 octane gas (including the gas pumped to fill up the tank at the very end of the trip.) Based on the odometer reading (cross checked with OBDII) the total distance driven was 738 miles.

Calculations: Since the EV range of the car (to drain the battery from 100% SOC to 10% SOC) is approximately 47 miles, the estimated miles on this trip powered by the battery was 28x47/90 = 14.62 miles. Subtracting this from the trip total distance, we get the gas-powered miles to be 738-14.62= 723.38 miles. Diving this by the 14.464 gallons of gasoline used, we get a gas-only mileage of 50.01 mpg

Conditions: The trip was mostly at highway speeds between 65 and 75 mph. As an older driver who has been driving EVs and PHEVs for over 5 years, I have gotten into the habit of accelerating and decelerating slowly. Except for an hour in the early morning one day, heating was not used, and given the temps in the low 70s, AC was not used either. The route took us from near mean sea level up a 4200 ft pass down to near MSL, then up a 1200 ft hill and then down close to MSL. All of this was repeated on the return trip. So not totally flat terrain. Unlike my Utah trip, I did not drive at speeds above 75 mph, and the AC was not used at all. Also, we had less luggage this time.

 

[Reyn]

@MrFixit or others. When my gas tank was full, CarScanner reported that there was 6.4 gallons in the tank. I had to drive a lot for that number to start dropping. Is this normal behavior? The horizontal line at the center of the car’s gas gauge corresponded to approximately 3.5 gallons remaining (as reported by Car Scanner.)

 

[MrFixit]

When my gas tank was full, CarScanner reported that there was 6.4 gallons in the tank. I had to drive a lot for that number to start dropping. Is this normal behavior?

Sorry, I have no experience with the I have no experience with reported fuel level.
It seems like you have demonstrated that there is some issue, but I wouldn't know how to resolve it.

 

[AHolbro1]

6.41 to be more precise. My suspicion is that represents the high point of the float, or sensing device. Getting a fuel level sensor tuned "just so" such that the extreme top of its movement coincides with the maximum amount of fuel in the cell AND also have it reliably indicate at the low end of the spectrum is a mildly difficult equation with limited/no upside. Who has complained that their car stays on "Full" for several miles after topping up before the level begins dropping? OTOH, miss on the low side and have people dropping off out of fuel all across the country and you'll not be selling many cars going forward. Consequently, attention in engineering, verification and validation testing are generally concerned with the low end and ensuring that sufficient warning is given to preclude fuel starvation, with only cursory attention given to performance at the top of the range.

The first time I topped up my 2019 Clarity, acquired on July 7, 2022, I put 7.15 gallons in, according to the pump. IIRC, I believe it was still predicting 5 or so miles of HV range at the time, with about 20 EV left on the GOM. Since I acquired the adapter and Carscanner app, I add the gallons reported by the pump to the value remaining according to Carscanner and have concluded the capacity is actually somewhere close to 7.5 gal (US) and this pans out fairly well when applied to a spreadsheet I keep on my daily commute for predicting fuel-burn.

 

[Reyn]

6.41 to be more precise. My suspicion is that represents the high point of the float, or sensing device. Getting a fuel level sensor tuned "just so" such that the extreme top of its movement coincides with the maximum amount of fuel in the cell AND also have it reliably indicate at the low end of the spectrum is a mildly difficult equation with limited/no upside. Who has complained that their car stays on "Full" for several miles after topping up before the level begins dropping? OTOH, miss on the low side and have people dropping off out of fuel all across the country and you'll not be selling many cars going forward. Consequently, attention in engineering, verification and validation testing are generally concerned with the low end and ensuring that sufficient warning is given to preclude fuel starvation, with only cursory attention given to performance at the top of the range.

The first time I topped up my 2019 Clarity, acquired on July 7, 2022, I put 7.15 gallons in, according to the pump. IIRC, I believe it was still predicting 5 or so miles of HV range at the time, with about 20 EV left on the GOM. Since I acquired the adapter and Carscanner app, I add the gallons reported by the pump to the value remaining according to Carscanner and have concluded the capacity is actually somewhere close to 7.5 gal (US) and this pans out fairly well when applied to a spreadsheet I keep on my daily commute for predicting fuel-burn.

Thank you very much. This makes a lot of sense. Thank you.