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June 19th, 2007

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
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 :P

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 :)

By KaiRo, at 20:49 | Tags: cars, physics, units | 2 comments | TrackBack: 0

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