MPG from car vs actual calculation

We just got home from a nearly 4,000-mile road trip in our Clarity. I have found that it's basically impossible to calculate true MPG. I kept the car in HV mode the entire trip. I noticed on many occasions that both the HV and EV lights were on and the power meter had a substantial amount of blue. Without knowing how long the car ran in those situations (not just downhill but on level roads), there really is no way to figure actual MPG.

 

The way to measure is to buy an OBD-II tool so you can track engine time and mileage during that time. If you don't care to do that, then just relax and drive the car.

 

I agree.

 

What are the units on "true MPG"?
It seems like it should only be miles and gallons.
It's only when you add energy to the equation (with a plug) that it becomes troublesome to calculate a reasonable number. Then you need to calculate a different number. Energy/distance or $/distance or maybe MPGe (but I'm not a fan). I personally find cost/mile useful and I calculate that both for all electric and for HV mode.

If you started with a full charge and then drove 4000 miles in HV mode and used 89 gallons of gas but kept a full charge (because of HV mode) then how is the MPG anything other than (4000/89) = 44.94 MPG.

If you're looking for some other number, (engine hours per 100km?), then fine but don't call it MPG

 

If you ever measure based on 89 gallons, you've already screwed up because you forgot to calculate at least the last baker's dozen fills. Might as well not measure at all at that point.

 

So how many gallons of gas did you burn? And what was your Ev charge when you got home?

Sent from my SM-G955U using Inside EVs mobile app

 

Its not a reliable number beca

ever think the pump at the gas station is wrong????

 

I don't understand the math you're using.

Miles/Gallons = MPG

A 4000 mile road trip should use something on the order of 89 gallons if done in all HV mode (at 45 MPG)

At the same time, it's both absolutely accurate (both miles and gallons are easily measured) and useless once you start adding energy to the equation (via a plug).

I think we need a measurement better than MPG, but I don't know of one that makes sense and that people could readily understand.

I use cost/mile but preface it by saying that the numbers are based on the cost of electricity and cost of gas.

But I can tell people that "when using electricity (@ $0.12/kWh), my cost/mile is 70% to the same car using gas (@$2.60/gal).
Generally, people can understand that

 

But...

While driving in HV regen will be putting energy into the battery. The car constantly switches between HV and EV while in HV mode. So you travel some distance but the battery is supplying power for some percentage of that distance depending on deceleration and hills. So you might say the "car" gets a certain Miles/Gallons but you can't say the ICE gets that mileage.

If you plug-in you'll be adding power to the car independent of the gas.

There are two complications. 1) There are two power sources at work whereas in an ICE car there is only one so it is straight forward. 2) The engine isn't driving the wheels directly (or at least not very often). Rather the engine is running a generator which is powering the electric motor to drive the wheels, but also sending some power to the battery. Furthermore while the ICE is powering the drive motor the battery may be helping.

I just think mpg isn't measurable with this car.

 

Given the number of electric plans in Texas that have free nights, I wish I saw a lot more PHEVs and BEVs around... A good 50%+ of my miles so far have been free for fuel cost.

I say you're doing it wrong if measuring 89 gallons because you've averaged the whole trip, and now have no sense of where mileage was bad or good, so you can't attempt to get better the next time you make that trip. Calculate mileage on every tank.

 

All power comes from either Gas/ICE, Plug/Battery, Gravity (downhill), or Tailwind (did I miss any?). To me, it doesn't matter if the ICE generates power and stores it in the battery and it's used later. It's still power from gas.

The calculation for MPG doesn't care, of course. It must be an average over some amount of distance using some amount of fuel.

Over an arbitrary distance, MPG is measurable, mostly (though when in electric mode, the MPG calculation throws a divide by zero error), but is less useful once you start combining multiple power sources.

But average MPG is a useful figure to know for longer trips: If I can't plug it in for this trip of X miles, how much gas is it likely to consume? Do I need to stop for fuel at this exit? Those are practical questions where having a number is better than not having a number.

 

It's only on a longer trip (where you don't plug in at all) that you can even get a useful number out of MPG. MPG must be an average over some distance. Shorter distances make for less useful numbers, IMO.

I've never had a tank of gas that didn't have mostly electric miles on it, so on a per-tank basis, the MPG number is useless.

Free nights would be nice No time of use plan here in Ohio.

To me, cost/mile (for different modes) is still one of the more useful figures. Especially talking to other people.

 

I used roughly 79 gallons over a trip of 3964 miles, which comes out to 50.18 MPG. However, I did top off the battery several times (when free charging was available). If I had not, the battery would eventually gone down to nothing because, try as it might, the car never fully adds back any battery it uses while in HV mode. Also, a lot of the trip was in very mountainous areas with some upgrades of 6-8%. There were far fewer downgrades of that percent that were as long as the upgrades, so it became a negative sum game with the SoC at day's end.

But, even without any charging, the only approximation of MPG is the simple formula of miles/gallons which is what the car shows you on the info screen. And yes, I did have the HV range fix done before the trip.

 

I did a trip in early Sept and for most segments, car was run only in HV mode from the time the tank was filled until it was again refilled.

I drove typically between 65mph and 70mph (I like to get good fuel economy even at the expense of taking a little longer to get to where I am going).

The highest mileage reported by the car trip computer was about 61mpg, but the calculated mpg (amount of gas divided by miles use) was about 56mpg (I forget, something around these numbers) but more typically, car reported 55mpg to 57mpg, but calculated by consumption, actual mileage was 51 or so, but it is important to state that a tank of gas in HV mode will not keep the battery in the same state of charge as the battery was in when the car was booted. On a typical tank of gas, the SOC would drop by 2-3 bars. Assuming that 2-3 bars is about 10 miles of electric range, then the calculated mileage from the pump would be a couple of percent higher than real MPG.

This though. Coming from my Gen 3 Prius, I would say that driven the same way, the Prius was lucky to get 50 mpg. This car almost always overstated mileage as well. Often the car would give 52-54mpg, but the gas/miles calculation would often give 48 to 50 mpg (driven the same way).

Now considering that the Clarity is far smoother and quieter than the Gen 3 Prius, and is just overall a much more refined vehicle, it is hard to complain about the calculated mileage not matching the car reported mileage and while the car reported mpg is more in error in the Honda, the Prius was always off too, though not quite as much.