MPG ratings for PHVs are stupid

I'm getting about 122.1 MPGe and 47.6 MPG. USB and I can now compare the true performance of our cars without the ever changing price of fuels clouding the issue. Now we can discuss the driving styles and route differences that allows his PiP to be more efficient than mine.

 

My figures are substandard compare to others.

ClearWater Blue PiP did 135 MPGe on electric miles and 60 MPG on gas miles.

Jeff N (switched cars with devprius) did 130 MPGe on electric miles and 65 MPG on gas miles.

 

I would inject that your comparison should also include your differences in cost of fuels. USB pays out the nose for electric which influences his charging strategy, driving styles, routes, etc ... and thus his choice of vehicle.

 

if you go to fuel economy.gov, you can plug in your own numbers including costs and it will calculate this stuff fro you.

 

I think the MPGe rating is really useful to people who truly want to go green, meaning using as little energy as possible to accomplish something.

If you have 2 cars, both having a 20mile range, but 1 car only requires a 3kWh battery, and the other requires a 6kWh, you can see where this is going. This would affect charging time, and people with solar panel installs would probably care about this # as well. MPGe basically tells you how efficient your EV is. This # doesn't matter to most of us (range is almost always the first # we want to know), but it can be useful, especially once EVs are more common.

 

In fairness, it is a new and evolving technology and EPA and DOT metrics will evolve also as they have with the regular vehicle MPG.

I would like to see EPA add an EV Range number so people can do the math for their situations.

Comparing Prius to Volt to Energi where each has different EV ranges and MPG's as hybrid.

 

They already have the range listed. It is immediately underneath the fuel efficiency and above the estimated annual fuel cost. New Fuel Economy and Environment Label

 

I must apologize for the long post.

MPGe considers 33.7 kWh of electricity to be equivalent as a gallon of gasoline, derived from the energy content. Since gasoline combustion is inefficient (39% max in Prius), the miles you get out of 33.7 kWh of electricity is about twice more than you would get from a gallon of gas.

Generating electricity from fossil fuel also require inefficient combustion at the power plant but that's ignored because EPA numbers are vehicle efficiency (nothing to do with fuel production). That's why I don't like MPGe. If you hook up a generator to Prius gas engine at the power plant and run Leaf with it, you'll get 99 MPGe, per EPA. However, a regular Prius is rated 50 MPG. That's how deceiving MPGe is.

Another issue is combining both the MPGe (electricity) and MPG (gas). This composite figure will be higher the more you use electricity due to the nature of inflated MPGe number. However, the low gas MPG can also drag it down. Generally, the plugin with a bigger battery will be able to better inflate the number.

That's why SAE recommended to separate both numbers and not combine them. You need the breakdown of both fuels to really see the efficiency of both. For my PiP, I am getting 128 MPGe (263 Wh/mi) on electric miles and 54 MPG on gas miles. No other plugin can match this efficiency breakdown nor the cargo space.

Choosing a plugin comes down to two fundamental questions. How much gasoline do you want to displace with electricity? How much efficiency and cargo space are you willing to give up to do it?

Looking at various plugin cars, we know the first 11 electric miles does not impact efficiency nor interior space. PiP is still rated 50 MPG on gas with no compromise in cargo space. It looks like the new Accord plugin (13 miles range) has 1 MPG lower in gas mileage and the trunk is smaller. Ford Energi plugins has 21 electric miles and 4 gas MPG lower (smaller trunk as well). Volt has 38 electric miles with 13 MPG lower than PiP and it gave up one rear seat with only a compact car interior.

The more you want to use electricity, the more you'll have to compromise (price included). Where do you draw the line? It'll depend on your driving distance/pattern as well as the charging pattern. I have pointed out before that you can mitigate EV range with opportunity recharges but you are stuck with the interior and cargo space.

EPA and SAE worked out a general number based on statistic so chances are that, you will be close to it but YMMV. PiP covers 29% of the miles with electricity. Ford Energi covers 48%. Volt covers 64%. The higher percentage you want to cover with electricity, the less interior space you'll have (and the lower gas engine efficiency).

Emission is another consideration. EPA included production of both gasoline and electricity to compute and they came out with these figures. PiP is rated 210 gram of CO2 per mile. Both C-Max and Fusion Energi are rated 240 g/mi. Volt is rated 260 g/mi. Leaf is rated 230 g/mi. A regular Prius is rated 222 g/mi. Those numbers are based on the emission weighted by kWh generated in all states in the US. The data is a bit outdated and the latest data shows there is about 5% improvement in the grid mix.

The reason I bought PiP is to accomplish two goals. Reduce emission and displace gasoline with electricity (40% of my miles so far). I am able to do both without compromising interior space and I still get a flat cargo floor. I just gave up a spare tire.

 

It shows both the electric and gasoline driving range. For blended plugins, EPA shows Electricity+Gasoline since both can be used as more power is needed. The breakdown of each fuel used is also shown.

 

Electricity comes from many other sources that fossil fuel.

 

Yes, it can come from many other sources. Currently, 2/3 of it is from fossil fuel.

 

oh god not this argument again

 

Scroll down. It is below Annual Fuel Cost. You can simply search for "Driving Range" on that page.

Oops, you were asking about the breakdown of the two fuels. It is in the Miles per Gallon Equivalent row.

Below is for PiP.

Electricity + Gasoline​

95 ​

Combined​

​

.2 gal/100 mi of gas plus​

29 kW-hrs/100 mi​

 

It is a valid point but when does it become diminishing return?

A 25 MPG midsize would use 6,000 gallons. 50 MPG Prius cut that in half, saving 3,000 gallon. PiP (29% EV) will cut 870 gallon on top of Prius. Ford Energi (48% EV) will cut 1,186 gallons on top of Prius. Volt (64% EV) will cut 1,540 gallons on top of Prius.

The gallon saving between these plugins are 354 to 670 gallons. That's it! Electricity usage (emission increase and charging time) between them are very noticeable.

Looking at the big MPGe numbers, one would think the difference in gasoline displacement for plugins are much greater than a regular hybrid. In reality, regular hybrids have the best bang for the buck. Plugins are incremental improvement, if designed properly. If not, it can actually increase emission.

 

90% of your post is off topic, I do want to address this on topic part though.
Do you find a screwdriver deceiving because is makes a rotten hammer?

MPGe is a wonderful tool as long as you use it for the job it was designed for.
It is there to give a comparison with gasoline based on energy content. Not a comparison of GHGs, or cost. Just simply how far you can go on the energy content of the electricity converted into gallons of gas.

It gives a common yardstick to allow people to compare different vehicles, or the same vehicle at different times.

If you want to compare GHG emissions MPGe is the wrong tool.
If you want to estimate fuel costs, MPGe is the wrong too.
If you want to compare the vehicle efficiency between different cars, MPGe is a good tool.