h2-ca Wiki
Page name: GuessOhMeter

The Miria Guess O Meter

The Mirai displays the predicted "miles to empty". This confuses a lot of drivers

Knowing where this number comes from can help eliminate a lot of the confusion. Put simply, this number is the MPGe of "your driving" over the last 8 fills times the number of KGs in the tank. The KGs in the tank is intentionally under-reported so that you have an 8% reserve.

So, for a gen-2, the tank is nominally 5.6 KG. If it is "full", this is 5.15 KG. The cars EPA rated MPGe is about 75, so this gives you a range of 386 miles.

Tank KG

The first fuzzy number is how much H2 is in the tank. Hydrogen is a gas and it is stored under pressure in the Mirai's three (2 for a gen-1) tanks. Gases under pressure roughly follow the "ideal gas law". Basically, this means that if you double the pressure in a tank, but keep the temperature the same, you have double the gas. Unfortunately, hydrogen is not an "ideal gas". So double the pressure is actually only about 1.7X the amount of fuel. Fortunately, the car "knows" how hydrogen behaves and takes the temperature and pressure together and gets are pretty accurate number for how much H2 is in the tank. Some vendors (ie, Hyundai), will display this as %full, but for some reason, Toyota just gives you the bar graph display.

MPGe for last 8 fills

This is a rolling average of the last 8 fills. So if you drive badly, your bad driving will not get reflected right away. The problem with this is that you can drive well or drive badly "on demand". You can decide to drag race everyone off the line, or mosey onto the freeway. Getting 30 MPGe or 70 MPGe on any particular trip segment is not that hard.

MPGe is more than driving

H2 is used to generate electricity. This electricity is used for everything in the car. Other than driving, the biggest user is cabin heat. Next comes air conditioning, seat heaters, and finally stuff like lights. If you idle and use cabin heat, you can easily push your MPGe average to very low values. After all, you are using hydrogen and not moving. If you need range, make the "other electricity uses" your first priority to curtail. Climate and driving modes control how the car generates cabin heat and make a big difference. Amazingly, AC is not nearly as big a user as the heater.

The range number

is just the KGs "available" in the tank times your MPGe. If you get a "somewhat short" fill and drive badly, this can be 5 KG and 40 MPGe. So your range will be 200 miles.

How the range updates

As you drive, the "miles remaining" updates based on your using fuel, and miles you have driven. The "used fuel" is likely a doppler flow sensor that measures the hydrogen on the way to the fuel cell stack. These sensors are really quite accurate. This is what is used to give you a live MPGe. This live MPGe will eventually become a part of 8 tank average, but not on this trip.

So if you start at 5 KG and 40 MPGe, your range will start at 200 miles.

You drive the first 40 mile leg and get 80 MPGe, so your first leg uses 0.5 KG. The car now has 4.5 KG and will report a GOM of 180 miles. But you drove 40 miles, so your tank range looks like 220 miles. If you continue to drive at 80 MPGe, you should be at 400 miles driven when the GOM finally says zero.

If you need to, you can ignore the GOM

If you need to get to a destination on this tank, you can adjust your driving to "make it". Say you are going from San Jose to LA. This is 320 miles. If you start with 5 KG in the tank, you need average 65 MPGe. The "miles remaining" does not really matter. Drive so that the MPGe for "this trip" is 65+, and you will make it. This is really useful if you are worried about getting fuel at Harris Ranch. By driving the first leg conservatively, you can make the fuel stop "optional". If HR has fuel, then top off and hammer down the rest of the trip.