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Old 05-07-2021, 07:18 PM   #1 (permalink)
Isaac Zachary
High Altitude Hybrid
Join Date: Dec 2020
Location: Gunnison, CO
Posts: 741

Avalon - '13 Toyota Avalon HV
90 day: 40.56 mpg (US)

Prius - '06 Toyota Prius
Thanks: 441
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Energy per distance, better than gallons/liters?

Ok, so there seem to be a million ways to compare gasoline/petrol with diesel with electricity with CNG with LP, etc. But what makes the most sense?

In the USA we're all used to miles-per-gallon. In some other countries, they use liters per 100 kilometers, which in some ways is a more intuitive measurement. The nice thing about both is that they aren't price locked. A 40mpg car will cost less to fuel than a 30mpg car, regardless if the fuel is $1, $3 or $100 per gallon/liter. But that's assuming we're talking about the same fuel, or fuels with basically the same price per gallon. If one car get's 100mpg, but it uses electrcity, and another gets 40mpg, but it uses diesel fuel, the price of each of these fuels could make the miles-per-gallon type measurements kind of useless.

Cost-per-mile remedies a lot of the MPG measurement problems. If my car costs 10 per mile and someone else's costs 5 per mile, then the comparison makes perfect sense regardless if the fuel is diesel, gasoline or electricity. You can also add more to the total cost-per-mile than just fuel costs. What if a vehicle costs more to insure even though it costs less to fuel? It could still be more expensive and using cost-per-mile would help differentiate that. But what if we're comparing the same or similar vehicles from different places? Differences in fuel costs could make the two vehicles seem like they are completely different when they aren't.

Another measurement is efficiency. But efficiency of what? The motor/engine? That plus the drive train? But that would seem to imply that the only important factor is motor/engine efficiency and that things like rolling resistance, acceleration or air drag have no apparent effect. So instead do we calculate what would be the absolute least amount of energy needed to move an average person through air, at a "highway speed," over a paved road and then compare that to the total energy the vehicle in question uses?

So what if we just did energy per distance. Calories per meter, joules per foot, BTU's per mile? My vehicle uses x amount of energy to go a set distance. That would make comparing between diesel, gas and electric vehicles a bit easier to understand. Then just state the cost per unit of energy, and we know how much it both uses and costs to drive. It may be a bit difficult to calculate since we don't always know the energy content of our fuels, like winter vs. summer blends. But at least we could guestimate and that would be better, in my opinion, since it would help alleviate the occasional complaint of "well diesel has more energy, and summer fuel has more energy and electric fuel has more energy".

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