The Mirai’s MPGe at 50 mph

The US Environmental Protection Agency (EPA) has estimated the combined city/highway miles-per-gallon-equivalent (MPGe) of the Toyota Mirai at 67 MPGe, and the range of its 5 kg tank as 312 miles.  Since neither of these numbers are terribly useful without knowing the speed and traffic conditions, I decided yesterday to get a better idea of how the MPGe depended on speed.

I’d already observed that the EPA’s 67 MPGe number was almost exactly reached on a trip on June 25 from the Saratoga station down US-101 to the Santa Barbara station, and that on average over the 1700 miles I’d driven the car so far I was getting 57 MPGe, due mainly to highway driving at the prevailing traffic speed of 70-80 mph on US-101 between Palo Alto and Pacific Grove.  What I didn’t know however was whether a higher MPGe was possible by driving at a steady 50 mph on country-road conditions unimpeded by stalled traffic and traffic lights.

So on the afternoon of Yellow Pig Day 2016 (July 17) I spent a couple of hours on various roads within 30 miles of Palo Alto with the cruise control set at 50 mph.   Being a hot day (85 F) I set the A/C to ECO and the climate control to 77 F, which was much more comfortable than with the A/C off.

The Mirai provides a number of gauges for assessing fuel economy, selectable using some of the buttons on the right of the steering wheel.   The one I found most useful (I’ll call it the TSF gauge) shows the Elapsed Time (HH:MM), Average Speed (MPH), and Average Fuel Economy (MPGe) for the selected trip meter.  There are four trip meters, A, B, P (my name for it), and ODO.  These can be selected by pressing the TRIP button on the steering wheel, and the corresponding miles-traveled is shown underneath the speedometer, independently of whether the TSF gauge is selected.  A and B can each be reset to zero by holding down the TRIP button on the steering wheel for two seconds.   P is reset to zero at Power-on, and ODO is the odometer.  For any of the four trip meters the product of miles-traveled and Average Fuel Economy on the TSF gauge gives the kilograms of hydrogen consumed since that meter was reset.

(It would have been more useful to have P reset automatically at fill-up than at Power-on, though resetting A manually at fill-up is almost as good provided you remember each time and don’t mind using up one of the two trip meters in this way.  With that trip meter, subtracting kilograms-consumed (as calculated above) from the tank’s capacity of 5 kg gives the remaining hydrogen.  I find remaining-hydrogen far more useful than the Mirai’s estimate of remaining range because the latter is based on the last eight fill-ups and is therefore not very useful when you can predict better than the car can what your likely driving pattern will be for the remaining tank contents.

After experimenting inconclusively on I-280 (too hilly to be informative), I settled on Foothill Expressway for my first experiment.  I started from Page Mill Expressway and ended a mile or so after passing under CA-85 where the 45 mph speed limit dropped to 40 mph.   After the first couple of miles the TSF gauge showed around 100 MPGe but then I started encountering red lights which soon knocked it down to the 70s.  Since the goal was to measure economy at a steady 50 mph I discarded that and decided that 50 mph on CA-85 would be easier to maintain.  So I went back to 85, and since the Saratoga hydrogen station was only six miles away I filled up there so as to start the measurement with a full tank, with the plan being to refill afterwards and compare the pump’s and car’s estimates of how much hydrogen had been used.

As usual I reset Trip Meter A at the station.  However to get a better idea of the MPGe at a sustained 50 mph I waited until I was on 85 and going at 50 mph before resetting Trip Meter B.

My plan was to drive to Gilroy and back, a round trip of 70 miles.  However at Cochrane Road, 21.8 miles, I could see stalled traffic ahead so I cut the trip short, took the Cochrane Road exit, and returned to the Saratoga station.

At Cochrane Road the MPGe was 99.6, which blew me away.  I figured the trip must have been downhill some of the way for the MPGe to be so high, and therefore expected the return to be lower.   So I was enormously surprised to see it actually increase on the way back, reaching 100.9 MPGe as I pulled into the Saratoga station 0.7 miles from the exit off CA-85.

My Mirai’s odometer is consistently showing 3% less distance traveled than what Microsoft Streets and Trips (one of the better navigators for PCs) claims. So going by S&T my real MPGe should be 104 MPGe. Not that there’s a big difference there when compared with the EPA’s city/highway figure of 67 MPGe, nor with my own result of 56.6 MPGe averaged over 1800 miles of lead-footed keeping-up-with-traffic driving since acquisition.

When I filled up on the return to Saratoga the pump showed 0.446 kg. The car judged the round trip at 46.0 miles (the 45.0 miles on the gauge plus 1.0 miles from the station to where I reset Trip Meter B on reaching 50 mph on CA-85), so the 100.9 MPGe figure means that the car thought it had consumed 46.0*100.9 = 0.456 kg, a difference of 0.01 kg or 1/3 oz (and .01/.446 = 2.2%) more than the pump’s reading. This difference is well within the pump’s claimed accuracy of 5%, but it’s only 0.2% of the tank’s capacity and therefore insignificant. I’ll start keeping an eye on the car’s MPGe number in future to get a better idea of car-pump agreement on longer trips.

On the social-interaction aspect of driving at 50 mph in the rightmost lane of a 65 mph freeway, I have only two observations. In the three hours I spent on these experiments, only one motorist expressed any arguably overt hostility and that was a taxi driver who briefly honked at me as he passed. (And for all I know he was just appreciating my PROTON POWERED license plate. Honking doesn’t offer quite the same explanatory bandwidth as Tolstoy’s War and Peace.)

What I did find however was a number of leeches that attached themselves almost to my rear bumper, each for several miles. My first thought was that this was how they complained, and my blood pressure went up a bit. But then it occurred to me that maybe some motorists are happy not only to drive slowly for better mileage but in the draft of another slow driver for even better mileage. This thought was reinforced by the observation that sometimes I had a caravan of several cars tightly anchored to me, none seizing any of the frequent opportunities to merge left into a faster lane.

So what does this mean for the Mirai’s so-called range? Well, if you believe Streets and Trips over the Mirai’s odometer, and you can avoid traffic jams and traffic lights, and that all 5 kg of the Mirai’s two tanks is available, and you aren’t experiencing major altitude changes, then by setting your cruise control to 50 mph and setting your A/C to a tolerable level and driving off into the sunset you should be able to cruise into the next hydrogen station after driving 520 miles. Or if you drive out and back from say Truckee or San Juan Capistrano with your cruise control at 50 mph you should be able to go as far as 260 miles (252 by the Mirai’s odometer until further confirmation) before turning around.

Suggestion to Toyota: a trip meter that resets at each fill-up. This could replace the one that resets each time you start the engine, which doesn’t seem terribly useful. Currently I use Trip Meter A for that but this entails resetting it at the station.

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