Having been driving in the US for over half a year now with a car that only sports metric units in the dashboard (kilometers-per-hour and degrees Celsius ONLY! ), I have had to get good at doing imperial miles-per-hour to metric kilometers-per-hour conversions on the fly while driving. Of course by now I have most of the important ones memorized, but every so often I encounter a less common one (most common usually being 25, 35, 55 and 65 MPH; less common being the ones somewhere in between). Luckily for me, I've found a really simple way to do the conversion - easy enough that a 4th grader could probably do it in seconds.

During my first days in the US, I wrote out a conversion chart on paper to keep on my dashboard for quick reference. This worked great for the first day - until the sunset and I could no longer see my cheat sheet

So then I figured I'd either have to memorize the conversion chart, or I'd have to get really good at doing the conversion in my head.

The more precise conversion rate is something around 1 mile ~ 1.609344 kilometers. And for all practical purposes, let's just say that the conversion factor from miles to kilometers is 1.6. That is, if you are presented with a speed limit of 35 MPH, you would do 35 x 1.6 = 56. And you would know that 35 MPH ~ 56 Km/h.

Of course multiplying by an odd decimal number such as 1.6 doesn't necessarily come naturally. So for really quick and dirty approximations, I started by using a multiplier of 1.5. This made things a lot easier and it was safe in the sense that I was sure to never be speeding - but of course, I never felt efficient in the sense that I knew I could be driving faster - I just didn't know by how much. (Unless of course I expended a little more energy to actually calculate the 1.6 multiplier).

And then I thought about it a little more and realized that 1.6 could be thought of as 16 just by shifting the decimal place. This is useful for people for software and electrical engineers who are familiar with the fact that a lot of software/electronic quantities are often given in hexadecimal. So we are fairly good at spouting off anything that is 2^n. The key here was to note that for each whole 10's value in the speed limit, you would sum up that many 16's. And if there is a remaining one's value (in the US, this will always be 5 if it exists), you would just add half of 16, or 8.

Thus, if you saw 55 MPH, you would count up 5 x 16, which is is 32 + 32 + 16, which is 80. And of course we can't forget the remaining 5, which means you add an 8 for a total of 88. And you now know that 55 MPH ~ 88 Km/h.

But wait - that's only useful if you are familiar with your multiples of 16. That's still not easy enough for a 4th grader.

So to make things EVEN easier. We take a seemingly less direct approach and first divide the posted MPH speed limit by 5. So again using 55 MPH as our example, we get 55 / 5 = 11. And we're nearly there. Again since everything was based on 1.6, which became 16, we now maintain those factors and use 8 instead since we've just divided by 5 earlier. So multiplying our result by 8 we are now back at 88 Km/h.

Now this is easy! Especially since all posted speed limits come in multiples of 5, so it's child's play to perform the first division. And 8 is also a low enough factor for most children to recite multiplication tables from memory.

But when we look back, we see that we're actually right back to the original 1.6 multiplication factor. Only, we've taken 1.6, converted it to a fraction, 16/10. And then we reduce that fraction to 8/5. So now we have our conversion as MPH x 8/5. We then rearrange the operations to get MPH/5 x 8, which is the same thing.

Of course, I suppose in most cases, your car probably already has dual unit markings... moreover with the accessibility of in-car navigation systems, it's probably easier just to switch your GPS device to imperial units once you cross the border...

But if you need an excuse to have your kids practice their 5 and 8 multiplication tables, have them convert the speed limits for you