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I come across all types of things as I’m browsing internet classifieds for my next four-wheeled mistake. Even if I’m just looking for a stick BMW, the nature of online car sales platforms means the search results show listings that I don’t even want to see because they might be tangentially relevant or strategically tagged. Recently, I came across a cop-spec Chevrolet Tahoe and noticed something funny in its gauge cluster: The speedometer said, “certified.” Hm! With a quick search, I found that nearly every cop car since the ’70s has a certified speedometer, even police Harleys. So what does certified mean, and who does the certification? I researched how these devices are made and I’m here to share my findings.

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When a speedometer is certified, it means that it reads absolutely accurately with a minimal margin of error. We contacted the Michigan State Police (MSP), which regularly conducts comprehensive police vehicle testing, for a little more insight.

“A certified speedometer is one that the manufacturer guarantees to read within a specified range of +\- 2 mph of true speed at any speed,” Lt. Michael McCarthy in an email exchange.

Aside from radar and laser speed guns, I thought cops visually estimating speed was dubious at best, but the certified speedo does add a layer of measurement, especially for pacing other cars. The MSP uses its speedos to verify radar readings and pace vehicles.

Image: Ebay listing

The speedometers are certified by the manufacturers these days, as Ford and Dodge sell their police vehicles as complete packages to departments across the country, instead of cars being bought as strengthened base models and converted locally. Back in the days of absurd mustaches and disco, I think small independent calibration shops would do the work. But police departments now make sure that everything is accurate in-house using chassis dynamometers. Lt. McCarthy told me that the MSP verifies its certifications on a chassis dynamometer at speeds of 35, 55, and 70 mph and only reverifies the speedometer if the vehicle has had any drivetrain work done to it.

Cop speedometers should be pretty easy to calibrate, with one wheel and tire package used for the life of the vehicle. With predetermined gearing, accurate road speed can be calculated with precision. It’s also important that it’s road speed and not GPS speed, as GPS doesn’t factor elevation in its speed calculation.

We reached out to the big three makers of police vehicles, Ford, General Motors (GM), and Stellantis, and received mixed answers. Ford never got back to us, GM is still digging for info about its in-house certification procedure, and Stellantis gave a buttoned-up PR response.

“We can’t comment on such actions by a customer,” Stallantis said in an email. “Suffice to say we, like every automaker, maintain internal standards and procedures to deliver accuracy.”

Although cop speedometers are certified, normal consumer cars are not held to the same standards. They don’t need to be, and most are actually 5-15 percent off from actual road speed. This is because cars come from the factory with different wheel and tire options that change the diameter of the wheel and tire. In simple terms, this causes the wheel and tire assembly to rotate faster or slower, which biases the speedometer faster or slower. Also, it isn’t a huge deal if the speedometer in your car is off by a couple of mph, so OEMs don’t necessarily bother calibrating all of them with different transmission speed sensors or changing the gauge readout.

But next time a cop starts pacing you on the highway, just know that their speedometer readout is totally accurate and admissible in court. It’s one of many tools of highway speed enforcement, and I had no clue it existed until now.

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