What Happens When You Cross the US-Canada Border
I drove from Seattle to Vancouver for the first time in 2019. The moment I crossed the border, every speed sign changed from miles per hour to kilometers per hour. The highway limit went from 70 mph to 110 km/h. My brain interpreted "110" as "dangerously fast" and I dropped to a crawl — briefly causing a traffic hazard.
70 mph and 110 km/h are essentially the same speed. I knew that intellectually, but the number felt wrong. That feeling is why unit conversion matters — not just as a math exercise, but as a safety reflex.
The Formula: One Constant
The United States is one of only three countries in the world that still officially uses miles per hour (alongside Myanmar and Liberia). Everyone else uses kilometers. The relationship between the two is exact:
The Conversion
For mental math purposes, 1.6 is close enough. Multiply mph by 1.6 to get km/h. Divide km/h by 1.6 to get mph. In your head: multiply by 8, the divide by 5. Or multiply by 3, then divide by 2. Pick whatever clicks.
Common Speed Conversions — Memorized
| Situation | MPH | KM/H |
|---|---|---|
| Residential speed limit (US) | 25 | 40 |
| School zone | 20 | 32 |
| City street | 35 | 56 |
| US highway | 55 | 89 |
| US freeway | 65–75 | 105–121 |
| Canadian highway (typical) | 68 | 110 |
| Autobahn (advisory) | 81 | 130 |
| Human sprint (~world record) | 28 | 45 |
The most useful number to anchor on: 60 mph ≈ 97 km/h ≈ "about 100 km/h." It is close enough to be practical in every real-world scenario.
Why the US Never Switched (And Why Canada Did)
Canada converted to the metric system in 1970s as part of a deliberate national policy. The US started that same conversion process in 1975 with the Metric Conversion Act — and then quietly abandoned it because it was voluntary. No enforcement mechanism, no deadline, no economic pressure strong enough to overcome inertia.
The US aerospace industry uses both systems — which caused the 1999 Mars Climate Orbiter to crash after one team used metric units and another used imperial. The $327 million spacecraft was lost because of a unit conversion error.
The practical cost of the US staying imperial? American car manufacturers must produce speedometers showing both units for export. GPS apps must switch display units at borders. And approximately every American who has driven in Canada has had the same momentary panic I had on that Seattle-Vancouver drive.
Quick Mental Conversion Tricks
Multiply by 1.6
Or: double it, subtract 20%. 60 mph → 120 → 96 → ≈97 ✓
70 × 1.6 = 112 km/h
Multiply by 0.6
Or: take 60% of it. 100 km/h → 60 mph ✓
110 × 0.6 = 66 mph
The Fibonacci sequence also works as an approximation: consecutive Fibonacci numbers (5, 8, 13, 21, 34, 55, 89...) happen to be close to mph/km/h pairs. 55 mph ≈ 89 km/h. 89 km/h ≈ 55 mph. 34 mph ≈ 55 km/h. It is a genuinely useful coincidence.
Frequently Asked Questions
Is 100 km/h the same as 60 mph?
Close but not exact. 100 km/h = 62.1 mph. For practical driving purposes — like interpreting a Canadian speed limit — treating 100 km/h as "about 60 mph" is accurate enough. The 2.1 mph difference is within the margin of speedometer error on most vehicles.
Why does my GPS show different speed limits than the signs?
GPS speed limit data comes from map databases (like HERE or TomTom), which can be outdated by months or years. Speed limit changes from construction, new regulations, or local ordinances take time to propagate to commercial map data. Always defer to physical signage over GPS displays.
What about knots — how do those compare?
1 knot = 1 nautical mile per hour = 1.852 km/h = 1.151 mph. Knots are used in aviation and maritime navigation because nautical miles align with the Earth's coordinate system (1 nautical mile = 1 minute of latitude), making navigation math simpler at scale.