move over MPGs, here come MPDs - GreenHybrid

stevejust · #1 (**permalink**) 02-06-2008, 03:43 PM

Quote:

For decades, miles per gallon — mpg in its familiar shorthand — has been the only way for consumers to understand how much gasoline is going into their tank, and what it really costs. But mpg isn't cutting it any more.

Like leagues, fathoms and pecks, mpg has become a relic, a unit of measure that has lost its meaning. It's a quaint reminder of the days when a gallon of high-test cost a buck or less, and Jimmy Carter donned his fuzzy cardigan and kindly asked everyone to crank down the thermostat and conserve energy.

It's time for new rules, and new math: Miles Per Dollar, or mpd.

This isn't actually as disappointing as I thought it'd be coming from MSN Autos.

http://editorial.autos.msn.com/artic...&topart=luxury

spinner · #2 (**permalink**) 02-06-2008, 06:04 PM

And if the cost of fuel goes way down, everyone should get off scot-free, right? This writer doesn't understand neither mathematics nor economics. He can toss off an idea but can't see it from more than one angle.

But he's sort-of on the right track: Should we be thinking in terms of miles per gallon or in gallons per miles (or gallons per 100 miles)? I guess one's more meaningful than the other depending on whether you're a farmer or a travelling salesperson. But if you endorse inverting the ratio, then now's a good time to start using Metric.

HybridVue · #3 (**permalink**) 02-07-2008, 06:04 AM

MPD's add one more variable to the equation - the price of a gallon of gas. While I can control my MPGs by my driving style, I have less control over the price of gas. Certain states will always have an advantage. New Jersey gas will always be cheaper than New York by upwards of 20-25 cents per gallon. San Francisco will always have some of the highest gas prices in the lower 48. So the MPD's of two similar vehicles will vary based upon the cost of gas in your location.

With that said - I compute my gas cost per mile (CPM)- I currently average 8.7 cents per mile for gas. It is useful to track rising prices CPM but you can also track it by the price per gallon of gas. There really is no correlation between the cost of gas and my mpg. Either way I drive smooth

- regardless of the price of gas.

What I do take from the article is the ease at which we all swipe our credit cards to pay for not only gas but everything. Our world makes it way to easy to swipe now and hope to pay it off later. It would hit home more if you had to dig into the wallet and pay with real dollars. Then we all would see and feel the sting in our pocketbooks.

KenG · #4 (**permalink**) 02-07-2008, 10:15 AM

I think there is a good idea here but the author of the article missed it. MPD becomes the critical measure when we start to add plug in hybrids, full electrics, natural gas powered and potentially even hydrogen powered cars in the mix. I can envision a plug in charger in the garage that measures electric cost so you can add that to the gas bill at the end of the month and figure the integrated MPD for the month.

kdhspyder · #5 (**permalink**) 02-07-2008, 01:33 PM

Quote:

Originally Posted by spinner

And if the cost of fuel goes way down, everyone should get off scot-free, right? This writer doesn't understand neither mathematics nor economics. He can toss off an idea but can't see it from more than one angle.

But he's sort-of on the right track: Should we be thinking in terms of miles per gallon or in gallons per miles (or gallons per 100 miles)? I guess one's more meaningful than the other depending on whether you're a farmer or a travelling salesperson. But if you endorse inverting the ratio, then now's a good time to start using Metric.

Yes Gallons/100 miles driven ( GPC ) is far more useful. It puts the responsibility where it belongs on us the drivers. It's a more serious metric, not some ephemeral game ( what's the best mpg I can get? ). When one vehicle requires 2.1 GPC and another requires 5.5 GPC the differences are just as obvious as the current system. However the math of fuel usage seems easier to me. Fuel is $3 / gal and I know I have a 2 GPC vehicle = $6 / 100 mi driven. This would start to make us begin thinking on a fuel usage perspective.

The outdated mpg metric as noted in the MSN article is almost an encouragement to drive more; if one has a 20 mpg vehicle but suddenly begins driving a 50 mpg vehicle one can see this as an encouragement to get in and use the 30 'free' miles.

talmy · #6 (**permalink**) 02-07-2008, 02:42 PM

MPG is a bad metric because someone going from 10 to 12 mpg, a 2 mpg or 20% increase, will save the same amount of fuel as one going from 30 to 60 mpg, a 30 mpg or 50% increase. Much better to express in terms of gallons per 100 miles since distance traveled tends to be constant and the gallons vary. In that case change from 10 to 12 becomes 10 to 8 1/3 and the change from 30 to 60 becomes 3 1/3 to 1 2/3, or 1 2/3 for each.

The mpg metric makes it appear that Hybrids make the most sense for small cars, but in reality the most fuel would be saved in large vehicles (think taxis, local delivery trucks, busses).

And if you want an even better metric, try gallons/year/person. That's what really counts!

(And in the spirit of the orignal post, dollars/year/person).

bwilson4web · #7 (**permalink**) 02-07-2008, 03:13 PM

Serious studies of transportation cost use dollars per mile, which is one reason why the bogus CNW Marketing study received ill deserved attention. When you look at the fleet studies:

http://avt.inl.gov/hev.shtml

You find the summary reports use terms like:

Operating Cost:
. . .
Maintenance Cost: $0.03/mile
Operating Cost: $0.08/mile
Total Ownership Cost: $0.29/mile

These are much more practical numbers but I agree, it tends to mask the individual vehicle performance. So they go on to report:

Operating Performance:
Total miles driven: 68,287
Cumulative MPG: 47.10*

It comes down to whether you want metrics that force 'long division' to calculate expected fuel burn or a simpler, multiply. The "<quantity>/mile" rating supports multiplication to plan a trip, which is much easier than long division required with "miles/<quantity".

Bob Wilson

rrrrrroger · #8 (**permalink**) 02-08-2008, 03:14 AM

Quote:

Originally Posted by kdhspyder

Yes Gallons/100 miles driven ( GPC ) is far more useful. ..... When one vehicle requires 2.1 GPC and another requires 5.5 GPC the differences are just as obvious as the current system.

The flaw with that system is that the numbers are less variable. For example:

0.4 gallons/100 miles (VW 2-seater)
versus
1.4 gallons/100 miles (insight)

Those numbers don't sound very impressive to the buying public. There's really not much difference, and the average customer would go "eh" and be underwhelmed. But if you say:

240 mpg (VW 2-seater)
versus
70 mpg (insight)

Now that sounds impressive! The numbers are significantly bigger and make people go "wow" when they hear them. And thus I'd rather stick with MPG because it's easier to sell, which means it's easier to convince people to upgrade to more-economic cars.

bwilson4web · #9 (**permalink**) 02-08-2008, 05:17 AM

Quote:

Originally Posted by rrrrrroger

The flaw with that system is that the numbers are less variable. For example:

0.4 gallons/100 miles (VW 2-seater)
versus
1.4 gallons/100 miles (insight)

Those numbers don't sound very impressive to the buying public. There's really not much difference, and the average customer would go "eh" and be underwhelmed. But if you say:

240 mpg (VW 2-seater)
versus
70 mpg (insight)

Now that sounds impressive! The numbers are significantly bigger and make people go "wow" when they hear them. And thus I'd rather stick with MPG because it's easier to sell, which means it's easier to convince people to upgrade to more-economic cars.

The 3:1 ratio remains the same regardless of the metric. I'm glad you spied the error.

Bob Wilson

gpsman1 · #10 (**permalink**) 02-12-2008, 08:59 PM

Here's a case to study. My actual results ( YMMV ):

So, is ethanol a good deal, or bad deal? It produces for sure lower MPG.
But the results are not obvious until you make another plot, cost per mile:

There is a very specific range, for me, where ethanol is a good value for the dollar, but it isn't obvious until you plot it out. -John