Why does the US still use mph instead of km/h?

Because the Metric Conversion Act of 1975 made metrication voluntary rather than mandatory, and the U.S. Metric Board created to coordinate the switch was defunded in 1982 before highway signage — the most visible, expensive piece — was converted. Every other major country either mandated the switch (UK is a partial holdout too) or had already used metric units before the automobile era.

Did the US ever try to switch to km/h?

Yes. In the mid-1970s several states piloted dual km/h-mph highway signs, and metric conversion was seriously discussed as federal policy following the 1975 Act. The effort stalled for cost and political-will reasons well before it reached national highway signage, and the 55 mph national speed limit of the 1970s oil crisis further entrenched mph in the public consciousness right as the metric push was underway.

Is the UK also non-metric for speed?

Yes — the UK uses miles and mph for road distances and speed limits despite being officially metric for most other measurements (it adopted metric for trade in 1965). Road infrastructure and signage are specifically carved out and remain imperial, similar in spirit to the US situation but for different historical reasons tied to the cost of re-signing the entire road network.

Does the US use metric units anywhere?

Extensively — science, medicine, the military, manufacturing for export, and food nutrition labels (in part) all use metric units in the US. The holdouts are almost entirely consumer-facing: road speed and distance, body weight and height in casual use, and cooking measurements. NIST itself has used the metric system as the U.S. preferred system since 1975; adoption in daily life just never followed.

Guide

Why does the US use mph instead of km/h?

The United States tried to go metric on the highway in the 1970s. It got as far as the signs before the law that funded it lost its teeth.

By Buğra SözeriPublished September 25, 2026

Every other G7 country posts speed limits in kilometers per hour. The United States posts them in miles per hour — and the reason isn’t that km/h is somehow unsuited to American roads. It’s a specific piece of 1970s legislation that made the switch optional, and an agency that lost its funding before the most visible part of the job — replacing every highway sign in the country — got done.

The 1975 law that almost changed the signs

The Metric Conversion Act of 1975declared the metric system the U.S.’s “preferred system of weights and measures” and created the U.S. Metric Board to plan and coordinate a national transition. Critically, the Act made conversion voluntary — Congress had deliberately stripped a mandatory-conversion clause from earlier drafts under industry pressure. Some states piloted dual km/h-mph highway signs in the mid-1970s as part of the early push. Then, in 1982, the Reagan administration defunded and effectively dissolved the Metric Board, ending federal coordination of the effort. Voluntary metrication, without a coordinating body or a deadline, simply stopped advancing in the one place ordinary Americans would have noticed it most: the road.

Why the effort stalled specifically on roads

Re-signing an entire national highway network is enormously expensive and highly visible — every driver would notice, and every complaint would land on a local official’s desk. Compare that to industries that quietly went metric already: pharmaceuticals, science, and much of manufacturing for export switched decades ago with essentially no public friction, because the change happened behind the scenes. Road signage is the opposite — maximum visibility, and a cost that has to be justified state by state. Once the federal coordinating body disappeared in 1982, there was no single entity left with both the mandate and the budget to push through the expensive, visible part of the conversion. For the deeper mechanics of why metric won everywhere else, see why metric units won everywhere except the US.

What actually is and isn’t metric in the US

Domain	Unit system used
Science, medicine, military	Metric (SI)
Manufacturing for export	Metric, widely
Road speed and distance	Imperial (mph, miles)
Body height/weight (casual)	Imperial (ft/in, lb)
Nutrition labels	Mixed (grams + Calories + some imperial)

This split explains a genuinely odd fact: NIST has treated metric as the country’s official preferred system since 1975, yet an American driver still reads a speedometer in mph. The law changed the preference on paper; it never funded changing the signs.

What this means if you’re driving across the border

For anyone driving into Canada or Mexico from the US (or the reverse), the mph/km/h split is not academic — it is the single most common source of accidental speeding, because the numbers on the sign and the numbers on a US-market speedometer’s dominant scale don’t match. Most North American cars display both scales on the speedometer face specifically because of this; see our guide on why speedometers carry dual km/h/mph markings for how to read the smaller scale correctly, and use a speed converter before a cross-border trip rather than doing the math at highway speed.

Frequently asked questions

Why does the US still use mph instead of km/h?: Because the Metric Conversion Act of 1975 made metrication voluntary rather than mandatory, and the U.S. Metric Board created to coordinate the switch was defunded in 1982 before highway signage — the most visible, expensive piece — was converted. Every other major country either mandated the switch (UK is a partial holdout too) or had already used metric units before the automobile era.
Did the US ever try to switch to km/h?: Yes. In the mid-1970s several states piloted dual km/h-mph highway signs, and metric conversion was seriously discussed as federal policy following the 1975 Act. The effort stalled for cost and political-will reasons well before it reached national highway signage, and the 55 mph national speed limit of the 1970s oil crisis further entrenched mph in the public consciousness right as the metric push was underway.
Is the UK also non-metric for speed?: Yes — the UK uses miles and mph for road distances and speed limits despite being officially metric for most other measurements (it adopted metric for trade in 1965). Road infrastructure and signage are specifically carved out and remain imperial, similar in spirit to the US situation but for different historical reasons tied to the cost of re-signing the entire road network.
Does the US use metric units anywhere?: Extensively — science, medicine, the military, manufacturing for export, and food nutrition labels (in part) all use metric units in the US. The holdouts are almost entirely consumer-facing: road speed and distance, body weight and height in casual use, and cooking measurements. NIST itself has used the metric system as the U.S. preferred system since 1975; adoption in daily life just never followed.

Sources & references

Authoritative references cited by this piece. Verified by Buğra Sözeri on the dates shown and re-checked at every deploy.

U.S. Metric Association / NIST — History of the Metric Conversion Act of 1975 — NIST's account of the voluntary 1975 Act and the 1982 defunding of the U.S. Metric Board(as of 2026-09-25)
NIST — U.S. Survey Foot / mile definitions and metric relationship — The exact mile-to-kilometer relationship underlying mph-vs-km/h road signage(as of 2026-09-25)
Federal Highway Administration — Manual on Uniform Traffic Control Devices (MUTCD) — The current U.S. standard mandating mph on public road signage(as of 2026-09-25)