Long trip fuel mileage methdology

I drove from the LA area to San Jose, and then back again on the same route (via I5) this past week. 355 miles each way (plus or minus a mile or two). Before each leg the car was filled at the pump using the automatic pump shutoff, and the MPG calculator in the car was set back to zero. Cruise control was set to the same values for each section of the route (to the extent possible). After returning home the car was filled up again at the same pump as before leaving. The ambient temp was around 65F in all 3 fills, and both stations were Valero's, but the pumps did not look anything like each other.

Amazingly, and perhaps coincidentally, the car calculated 50.6 MPG in each direction. It was not symmetrical though, because going north it worked up to a peak of about 51 MPG before getting to the central valley, and then slowly worked downwards, whereas heading south it started at around 49.0 and stayed in that vicinity until going up over the grapevine, then it slowly worked upward to the final value (the route is significantly downhill on that final leg).

So, given that one might expect fairly similar amounts of gas to be added on each end. Nope. In San Jose it was 6.471 gallons, whereas at home it was 8.555 gallons. Also on arriving in San Jose there were 3 bars on the fuel gauge, whereas there were just two when arriving home. I think most of this was probably just that the two pumps had very different "full" settings, but some of it might have been due to the fuel tank bladder doing its thing. In any case, the car's fuel economy calculation is pretty far off versus the distance/gallon value of
(355*2)/(6.471+8.555) = 47.251 MPG, that's about 7% different. Perhaps the two would eventually converge if I drove the same route over and over, but more likely the car's calculation is just a bit optimistic, they all seem to be.

 

Cruise control was set to 75 mph, when traffic permitted, in the central valley and on the less steep slopes. Subtracting pit stops the average speed for the whole trip was around 68 mph. (It was around 6 hours door to door, but I didn't time the pit stops, so estimated 45 minutes, and that gives 355/5.25 = 67.6.)

Going uphill, on the steep sections, the target was 65 mph. It was set that low not because I don't think a healthy Prius can go faster, but because the (original) battery is showing its age, even though it has not yet thrown any codes, and I didn't want to see the first HV battery related big red triangle in heavy traffic while climbing the grapevine. Going down the steepest sections the transmission was in "B", the headlights were on, and the ventilation fan set to its highest speed, all to try to redirect some of that regen current away from the slightly iffy battery. It still went to all bars set and green on the long downhill sections.

This is the first long trip I have made in this car. While it was the hands down winner in fuel economy, and the seat is very comfortable, by other measures it is not that great a highway cruiser. Compared to the last two "big" cars we have owned, a 2000 Subaru Legacy wagon and a 1998 Accord, the Prius is much noisier inside, more skittish on uneven pavement at speed, accelerates poorly, and it is more sensitive to cross winds. That is not a fair comparison though as those are midsize cars and this is essentially a compact. The highway manners of a Civic or Corolla would be a fairer reference. I used to have a Civic Hybrid, but never drove it on long trips, so I don't have any data to make that comparison.

There are a lot of Teslas on the streets at both ends of this trip, but I only saw a couple of them on the move during either leg of my voyage. Yet there were a bunch of them parked at a charging station near the grapevine, it looked like every slot was filled, here:

Google Maps

Presumably those were the ones which were about to go up and over.