Calibration

[Roymonster]

I recently got a ticket going onto the 309 expressway doing 81 in a 55 mph. But last time the radar/handheld was tested was 2/13/20 I recently got a ticket going onto the 309 expressway doing 81 in a 55 mph. But last time the radar/handheld was tested was 2/13/9 2015. And I know I wasn't doing 81MPH.

 

[single317dad]

https://forum.freeadvice.com/speeding-other-moving-violations-13/how-often-do-pa-state-troopers-radar-guns-need-checked-601338.html

https://forum.freeadvice.com/speeding-other-moving-violations-13/pennsylvania-radar-calibration-484386.html

https://forum.freeadvice.com/speeding-other-moving-violations-13/pa-radar-calibration-interval-193590.html

https://forum.freeadvice.com/speeding-other-moving-violations-13/radar-laws-pa-25205.html

https://forum.freeadvice.com/speeding-other-moving-violations-13/speeding-ticket-pa-343954.html

https://forum.freeadvice.com/speeding-other-moving-violations-13/radar-calibration-199167.html

ad nauseum

 

[justalayman]

your state, which you did not provide, makes a lot of difference in this. Some states basically have no requirement to calibrate. Others fairly stringent in their requirements.


but what the heck does this mean:

was tested was 2/13/20

Is your name Dr. Brown?

I recently got a ticket going onto the 309 expressway doing 81 in a 55 mph. But last time the radar/handheld was tested was 2/13/9 2015( and what is that date? 8999 years in the future?)

it's a good thing you didn't make it all the way to 88 mph

 

[quincy]

your state, which you did not provide, makes a lot of difference in this. ...

Roymonster tacked his question onto an old thread. This thread was created for him. The state is Pennsylvania, hence single317dad's links to all of the Pennsylvania threads that address this same question.

 

[FlyingRon]

I'm still trying to understand his date nomenclature. One appears to be four years in the future and the other appears to be last February (but I still don't know about the spurious 9 in it).

Note testing (typically done by the operator) and calibration (done by a specialized technician) are two different things.

 

[drruthless]

I recently got a ticket going onto the 309 expressway doing 81 in a 55 mph. But last time the radar/handheld was tested was 2/13/20 I recently got a ticket going onto the 309 expressway doing 81 in a 55 mph. But last time the radar/handheld was tested was 2/13/9 2015. And I know I wasn't doing 81MPH.

How do you know that.
When was the last time you had your speedometer calibrated.
Have you ever heard of speedometer error? That's the dirty little secret the manufactures don't want you to know about.
Right out of the box you could have a three to 10 mph difference in what thought your speed was and the speed the cop who pulled you said it was. That's why in drag racing they use Radar gun to measure speed. That's all irrelevant anyway ..81 in a 55 zone? You stuck out like a sore thumb, you're lucky you didn't kill someone! and even luckier that the Tack squad wasn't called out to blow you off the road before you did.

 

[Zigner]

Right out of the box you could have a three to 10 mph difference in what thought your speed was and the speed the cop who pulled you said it was.

I can attest to that. My BRAND NEW (~10 miles on it when I leased it) Toyota Camry is approx 2.5-3 mph off. 60 on the speedometer is about 57 in real life, so I won't be getting a ticket because of it...but it could just as easily have been off in the other direction. Toyota says it's considered to be within the acceptable specs...

 

[justalayman]

I can attest to that. My BRAND NEW (~10 miles on it when I leased it) Toyota Camry is approx 2.5-3 mph off. 60 on the speedometer is about 57 in real life, so I won't be getting a ticket because of it...but it could just as easily have been off in the other direction. Toyota says it's considered to be within the acceptable specs...

at least most errors are on the side of lower than higher, at least from what I have seen


I cannot find a requirement for motor vehicles in general but here is the CFR on requirements for motor carriers. I cannot imagine one addressing motor vehicles being any more stringent as it would make this one meaningless if it were.

49 cfr §393.82 Speedometer.
Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).

[70 FR 48054, Aug. 15, 2005]

so, you have plus OR minus 10%.

 

[single317dad]

I can attest to that. My BRAND NEW (~10 miles on it when I leased it) Toyota Camry is approx 2.5-3 mph off. 60 on the speedometer is about 57 in real life, so I won't be getting a ticket because of it...but it could just as easily have been off in the other direction. Toyota says it's considered to be within the acceptable specs...

Variances in replacement tires can cause this too. While this is certainly not the problem in your case with the new factory car, people who go to the tire shop and buy the same size tires often find that slight differences in actual measurements result in a speedometer anomaly.

Example: a car's original tires are 20" in diameter (10" radius). If the replacement tires are 10.5" radius (5% increase), then the circumference of the tires is increased by 10.25%. Therefore, for each axle rotation, the car travels 10.25% farther than the speedometer is calibrated for. 60 mph becomes 66.15 mph. Fuel mileage will also appear to reduce in inverse proportion, and the odometer will not be accurate.

I recommend all drivers occasionally clock their speeds at 60 mph for 60 seconds against highway mile markers to ensure speedometer accuracy. Some states, like mine, provide clocking areas on a few rural highways for this purpose.

 

[Zigner]

Variances in replacement tires can cause this too. While this is certainly not the problem in your case with the new factory car, people who go to the tire shop and buy the same size tires often find that slight differences in actual measurements result in a speedometer anomaly.

Example: a car's original tires are 20" in diameter (10" radius). If the replacement tires are 10.5" radius (5% increase), then the circumference of the tires is increased by 10.25%. Therefore, for each axle rotation, the car travels 10.25% farther than the speedometer is calibrated for. 60 mph becomes 66.15 mph. Fuel mileage will also appear to reduce in inverse proportion, and the odometer will not be accurate.

I recommend all drivers occasionally clock their speeds at 60 mph for 60 seconds against highway mile markers to ensure speedometer accuracy. Some states, like mine, provide clocking areas on a few rural highways for this purpose.

Totally agree. I owned a Suburban with oversize tires that had exactly the situation you describe above (I always estimated based on 11%).

 

[FlyingRon]

Sometimes the monster tires are a dealer installed option. My wife has bizarre tires on her Audi (high performance, low profile) that some tire stores won't even allow me to order because it doesn't match her car model.

Still, I've had people try the speedo calibration thing (going to the point of having it certified) but it doesn't often work in court here (Virginia).

 

[justalayman]

Example: a car's original tires are 20" in diameter (10" radius). If the replacement tires are 10.5" radius (5% increase), then the circumference of the tires is increased by 10.25%. Therefore, for each axle rotation, the car travels 10.25% farther than the speedometer is calibrated for. 60 mph becomes 66.15 mph. Fuel mileage will also appear to reduce in inverse proportion, and the odometer will not be accurate.

You may wish to reconsider those calculations.

 

[FlyingRon]

You may wish to reconsider those calculations.

Yep, if the radius increases by 5% the circumference also increases by 5%. I have no idea how 10.25% enters into it. 60 MPH becomes 63 MPH.

 

[single317dad]

You may wish to reconsider those calculations.

Yeah, I figured for area (pi r^2) rather than circumference (pi d). Geometry I was a long time ago. I originally had the numbers right, then thought "oh wait, that isn't right"...

 

[OHRoadwarrior]

Both my Prii are factory with a -2 mph error also. replacing the tires results in a variance of 1 to 2 mph. Ron, take your wifes Audi, get a flat stretch of highway, measure out the time for 5 miles, which at 60 mph would be 300 seconds, then factor in the number of seconds you counted. Or some GPS devices have a speed calculator that are within 1 mph. Some phone maps and apps will also calculate speed. We used the oldest method on semi trucks that had millions of miles on them when we noticed they did not appear to be governed correctly years ago.