Quick and Dirty Metric to Imperial Conversions (How to Entertain Yourself as an American Driving in a Metric Country)
I was driving in Canada recently, the fun part being all my car’s gauges are in imperial (i.e., Freedom 🦅) units (miles per hour, miles, etc..), but road signage is in metric units (Kmph,Km).
So I wanted to figure out how fast in miles per hour I should go in a given spot. The gauges are digital, so they can be made to read metric, but that’s too easy. Plus my car doesn’t let you switch units while driving and I kept forgetting to switch the units when I was stopped; so we’re stuck estimating.
I remembered from my running days 3 mi ~= 5 Km, so 10 Km/h ~= 6 mph. Which means for a known Km/h, Mi/h is given by
SPEED_MPH = (6*SPEED_KMPH/10)
If the speed limits in 100Km/h, then you should go 60 mi/h.
Using Google Map’s speed estimate, the estimated speed looked close. Note the estimate is often different from the car’s speedometer by 2-3 MPH so it serves as more of a sanity check than anything.
I then remembered the actual distance for5Km was closer to 3.1 mi. That means the quick and dirty estimate above works well at lower speeds, but the error accumulates at higher speeds like on a highway.
This error isn’t necessarily harmful since the estimate is lower, but could annoy other drivers.
A more accurate estimation method accounts for the tenths position:
SPEED_MPH = (6*SPEED_KMPH/10) + SPEED_KMPH//50
This gives a speed of 62 mph.
This estimate is really close since 100 Km/h ~= 62.1371mph. The trailing error won’t be noticeable until we near 1000+Km/h.
Probably not an issue unless your driving a supersonic jet and need to convert units on the fly.
In real life you should probably just pull over and change your gauges, but this kept me entertained for a little bit while driving.