2007-06-19 21:21

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# Tricky unit conversions

When I just read this entry in mrz' blog about fuel economy of his car (or driving), I immediately was left with a problem: How good is a value of 53.4 mpg?

The real problem is that around here in Europe, we don't measure mpg at all. Of course, we track fuel economy and we are measuring that, but mpg values are something you won't find. Not only that we're using kilometers and liters instead of miles and gallons - we actually don't care about the cruising range we have with a certain amount of fuel, we care about how much fuel the car burns to go a certain distance.

That's probably connected with the fact that we have gas stations every few kilometers, I never saw scary signs like "42 miles no service" on this side of the big pond, but I remember a few of those from California, Arizona and Texas. Because of that, less range is not the primary concern - but how much fuel we have refill after a 200 km drive is much more, esp. as a liter of fuel costs about a Euro (>.30) these days around here.

So, I needed to find out how many liters per 100 kilometers (l/100km) those 53.4 mpg actually mean. As a physics student, a good time to do some nice unit math.

So, a US mile is 1.609344 km and a US gallon is 3.785411784 l, which makes 1 mpg be .425 km/l - inverse that and multiply it by 100, and you get a formula of

(or just use 235, that's probably enough for every normal calculation).

The same is true for the other direction of the equation:

So, 53.4 mpg equals about 4.4 l/100km, which is indeed a nice value. Even though the car producers love talking about the 3-liter-car (which would make 78 mpg), usual cars are still in the 5-8 l/100km range (30-50 mpg).

Sometimes a number that sounds quite normal for someone in the US can leave us Europeans quite puzzled, as SI units are not that common in the US. Thank god there's unit math

Edit: Some people have suggested on IRC to use Google for the conversion, but as a science student, it's always good to know you can do it yourself and grasp the logic of such things

The real problem is that around here in Europe, we don't measure mpg at all. Of course, we track fuel economy and we are measuring that, but mpg values are something you won't find. Not only that we're using kilometers and liters instead of miles and gallons - we actually don't care about the cruising range we have with a certain amount of fuel, we care about how much fuel the car burns to go a certain distance.

That's probably connected with the fact that we have gas stations every few kilometers, I never saw scary signs like "42 miles no service" on this side of the big pond, but I remember a few of those from California, Arizona and Texas. Because of that, less range is not the primary concern - but how much fuel we have refill after a 200 km drive is much more, esp. as a liter of fuel costs about a Euro (>.30) these days around here.

So, I needed to find out how many liters per 100 kilometers (l/100km) those 53.4 mpg actually mean. As a physics student, a good time to do some nice unit math.

So, a US mile is 1.609344 km and a US gallon is 3.785411784 l, which makes 1 mpg be .425 km/l - inverse that and multiply it by 100, and you get a formula of

`x l/100km = 235.21458 / y mpg`

(or just use 235, that's probably enough for every normal calculation).

The same is true for the other direction of the equation:

`y mpg = 235.21458 / x l/100km`

So, 53.4 mpg equals about 4.4 l/100km, which is indeed a nice value. Even though the car producers love talking about the 3-liter-car (which would make 78 mpg), usual cars are still in the 5-8 l/100km range (30-50 mpg).

Sometimes a number that sounds quite normal for someone in the US can leave us Europeans quite puzzled, as SI units are not that common in the US. Thank god there's unit math

Edit: Some people have suggested on IRC to use Google for the conversion, but as a science student, it's always good to know you can do it yourself and grasp the logic of such things

Entry written by KaiRo and posted on June 19th, 2007 20:49 | Tags: cars, physics, units | 2 comments | TrackBack

Comments

Author | Entry |
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Chris Lawson | Further complicating the issue are the widely different energy densities of various fuels. Ethanol, for instance, contains only about three-quarters the combustion energy of an equal quantity of gasoline (petrol), which in turn contains about 30% or so less energy than diesel fuel. Both "miles per gallon" and "liters per 100km" are pretty broken measures of fuel efficiency when you take this into account. I blogged about this a while back if you're interested. |

Michael Lefevre | Even more confusing over here, where we are using miles for distances and fuel is sold in litres. Most people still seem to work in mpg (with a UK gallon rather than a US gallon), although l/100km is also quoted in places - but whatever you work in, if you want to calculate things based on how far you have driven and how much fuel you put in the car, you have to convert from miles per litre... 2007-06-19 23:03 |