Speedometer Verification

Have a GPS, or can you borrow one ?

 

I have a radar gun that I use and have found the speedo to be right on

 

You will need a calibrated reference if you plan on refuting radar.

Tom

 

You can ask your local police dept. who they use to calibrate their spedos.

 

First of all, the speedometer in my '10 II is just about dead on. I heard from another Prius Pilot in Canada whose VSS was also well within .5 MPH...only he expressed his findings in km/h. Those Canadians...
A GPS is useful on the interstate, provided you're at a fairly stable speed, and not going up and down large hills...depending on how accurately you want to resolve your land speed. Really....it's close enough!
Another method I've used is to run a long measured course with the mile markers on the interstate, and do a stare and compare with your odometer. The last time I did a 50-mile run, I (re)verified my VSS was within 0.5% Yes...the diggers and fillers do not place mile markers with differential GPS precision---but the inaccuracies work themselves out over a long measured run…and if they get the posts within a couple hundred feet of the true “mile mark” you should be within the accuracy range you’re seeking.
All of this will get you within about a half a percent of the true error rate. After that, you rapidly enter the realm of the nerds! There are a lot of Prius drivers there. They will probably opine on how you can get your speedometer to within 0.0001 percent of “true” speed.
IMHO, If you want to resolve your error beyond a half percent, you're going to have to consider variables like tire wear, VSS sensor variations, GPS inaccuracies, cartography, which WGS ellipsoid model is “reeeeeally” the most accurate, etc.... My education fails me at that point, but I'm not sure that a peer-reviewed study to resolve the speedometer error rate beyond 0.5% is really worth the mental gymnastics.

Good luck!

EDIT: LOL!!! I just reread your question...North Carolina...refute the radar....
Dude...you're screwed. Pay the ticket. Show up in court if you have the time, and you may get the points knocked off, LEO may not show for court, etc...but don't try the "street lawyer" angle of disproving the accuracies of doppler radar. Besides..If YOUR speedometer is off by 15-percent or even broken, you're still responsible for driving at the posted speed limit!
Good luck! (you're gonna need it...)

 

I verified my speedometer using two GPSs, and found it to be within .5 mph of the GPS readings... Most LEOs give at least a 5 mph grace...

 

But remember that speedometer and odometer errors are not the same.

By both industry standards, and by law in a number of jurisdictions, speedometers cannot read low. So they are generally set a little bit high, and mine is 1-2 mph high at highway speed.

In contrast, odometers cannot read [XXXlowXXX] high without inviting class action lawsuits over warranty fraud. Honda was hit years ago, Subaru was hit very recently (I have or had both). My Prius reads about 0.3% low, though that may change as the tires wear.

 

+++1 for GPS.

Over short distances or instantaneous speed readings your GPS may not be highly accurate. But for measured times (and distances) over ~ 10 seconds, GPS should provide highly accurate readings.

Have fun...

Keith

 

That's why I used the mile-markers to ground truth what the GPS was telling me. I'm not sure how a Prius knows how many miles it has been driven, or how fast it does so but I "presumed" that there's a VSS, or vehicle speed sensor that counts some rotating gizzywhopper or other. In my 2 motorcycles, it a magnet/pickup affair. The ECU (or whatever the manufacturer calls it) takes these pulses and resolves speed and distance, which is why when you change tire sizes, you also change the speedo and odo rates---making them more or less accurate. I say more or less accurate, since my Kawasaki's speedo (and odo) will go from a +5% error to being almost dead bang on simply by clubbing up a tire size.
Since I'm a noob to the land of Prius, I have NO FREEKIN IDEA how Toyota does the time and distance thing---but I can tell you that in the lands of GMC, Kawasaki, and Harley-Davidson....if your speedometer is off, then your odometer will be off by a corresponding amount!
Like I said earlier....I used the mile-marker test to validate the veracity of my GPS readings. Priuses....at least the one I drive, have pretty accurate speedometers and odometers....no matter how many components Toyota uses to arrive at these 2 values.

 

Any "verification" that you might do on your speedometer is not going to be worth anything in court as opposed to an officially certified radar device. In order for it to be admissible in court you have to have it authenticated, which would probably cost more than the ticket. More important is whether the Police Officer has had his equipment calibrated as required by the law in your state. You might be better off getting a lawyer. But it is not cheap any way you look at it.

As a young lawyer I went to court with a very nice lady on a radar ticket. I had read up on all the requirements and I asked every conceivable question. The Police Officer answered every question perfectly! He had the calibration certificate. He had been to the school within the time period. He had run the test as required. The upshot was that the Judge was laughing at my frustration and asked me "Counsel, what exactly is your defense?" I put on my most sincere face and answered: Your Honor...my defense is that this defendant is my 76 year old mother and she said she "I am not guilty I have never had a ticket in my life and this is why i paid for your law school!" That is my defense! The Judge and the cop both laughed, the Judge said I think that is a reasonable doubt, NOT GUILTY! I thanked him gathered my mom, took her out and we went off into the afternoon, at which time my mom said to me: "I told you I wasn't guilty!"

Try some variation of that defense. Watch the earlier cases and see if anyone wins. Good luck. You will probably need it.

 

lol, that is awesome! :cheer2:

As for the OP, just pay the fine and be done with it. Even if you aren't guilty, the cost and effort of fighting is not worth a simple fine. I believe the costs are capped at $500 in the US even if you were speeding in a school or construction zone so the penalty is pretty much nothing. I doubt you were going fast enough to get the max ticket if you are trying to weasel out of it with the 'ol inaccurate speedometer trick.

Just pay the fine and be done with it.

 

The speedo in my V-AT almost always reads 1mph below the RCC. It is also within 1mph of my Garmin GPS at least through 65mph.

 

My speedometer accuracy findings are:
Car's 50kmh is a real 46kmh based on radar and GPS reading comparison.
Car's 130kmh is a real 123kmh GPS based
Car's 190 kmh is a real 180kmh GPS based
So, the difference is not a linear one.

 

Some GPS units and software allow you to view the speed as monitored by GPS. Keep in mind it is a 1Hz signal, but it should work ok for these sorts of tests.

 

As 2k! said, most consumer level GPS units only update at 1 second intervals so you need to maintainin a steady speed for a few seconds

Use your cruise control on relatively level ground for the steady speed.

It also helps if you have someone with you to read the GPS speed to you while you are driving and watching the speedometer.