How accurate are car speedometers ? Is there a legal requirement for accuracy where you are ?
I don’t know how accurate they are. But I do know that if you’re ticketed for speeding, an inaccurate speedometer can be used as your defense.
So, to answer your second question, I would say no. Because wouldn’t it be the fault of the car manufacturer for a faulty speedometer? As opposed to your own fault?
Consumer speedometers are reasonably accurate. One of the car magazines used to have charts of displayed speed versus speed derived by using a calibrated radar gun. Police car speedometers, OTOH, are calibrated on a regular basis. So, if a cop says he clocked you going 80MPH, you were!
This is why the police usually give you a 5MPH margin when running radar guns. They know your speedometer is off a bit.
One thing to remember is that if you use significantly larger or smaller size tires your speedometer will be off.
Set your cruise control to sixty mph, and start your stopwatch at the next mile marker. When you reach the mile marker after that, you should be right at 60 seconds. The number of seconds faster or slower scales up to how many mph your speedometer is off when doing 60.
you could always set it at 55 mph and check it that way too 
60 mph isn’t the only speed you can check.
Also, i don’t advise you go 60 mph in a crappy car they jiggly and shake and oh my …
just htinking about it makes my head hutr.
too much alcohol…
Um, Trumpy, yeah. IDEALLY, the they do calibrate their radar guns correctly, and at the correct time/location/interval. But these are cops we’re talking about. If he clocks you at 80, chances are, you are doing 80, IF he’s got it calibrated and aimed correctly, there are no large reflective obstructions around, the ground is straight and flat, and there is no oncoming traffic.
Any obstructions, oncoming traffic, curves or dips in the road, of any kind, can and will throw off the radar. It’s not like airport radar that shows your exact location and heading. It just throws a speed up on the screen. Who or what’s speed it is, and if it’s accurate is totally up to the cop to decide. I, for one, have been ticketed for doing 67 when my speedometer said I was doing 60, and, oddly enough, my speedometer is five mph fast. I’ve had it professionally checked. So I was doing 55 and he got a reading on me of 67 and ticketed me.
Why did he get this reading? Sine-wave error. The road was sloped and curved quite a bit, there is no way I was headed directly at him at the time he took his reading.
Cops are fallible. Fight every ticket you get, especially if YOU don’t believe you were doing said speed. Remember, YOU were driving the vehicle, YOU know what you were doing. He was sitting a half mile away, parked, with a gun in his hand. How the hell does he know, for sure, what you were doing?
Fight the Power.
–Tim
The local Auto Clubs out here in California used to offer free speedometer calibrations. I would think that if you drove a lot on the highway that it is worthwhile to do.
It’s been a few years since I designed electronic speedos, but back then the electronic ones were good for + - 2% of full scale. For an 80 mph full scale that meant the speedo was accurate to + - 1.6 mph or a span of 3.2 mph. At that time they were calibrated so you were actually going slower at 55 than the speedo said. If the speedo said 55 you would really be going about 52 mph.
Of course it gets worse as the the full scale increases. For a 140 mph FS guy the possible error would be + - 2.4 mph or a span of 4.8 mph. If your speed is calibrated so all the error is on the high side, an indicated 55 mph could really be 50.2 mph.
The mechanical speedos were not as accurate, about + - 3% as I recall.
Now that handheld GPS units are cheap, and in the last three weeks, about ten times more accurate than before, the speed accuracy from one should be way under 1 mph. Use this to do your calibration.
On motorcycles, speedometers are notoriously inaccurate. Esp. the larger Japanese bikes will show at least 10% above your real speed when going fast (200 km/h or thereabouts). Notoriously, the Honda Blackbird wasn’t really capable of reaching its claimed 300 km/h speed, although the speedo would show something like 310 - 315.
When the Danish parliament passed a law where it could cost your license to ride at over 187 km/h on highways, a lot of people bought aftermarket digital speedometers, only to find that they hadn’t been going nearly as fast as they thought. Depressing, that.
Oh, and Homer: I understand your sentiment, and I would definitely fight it (and lose) if I was clocked unfairly. But if I got caught actually speeding (and that’s not too unrealistic), I’d say “Yes, officer” and pay my fine - it’s part of the rules as I see things. Don’t roll the dice etc.
A few years ago I happened to be in exactly this situation. The law (disclaimers – at that time, in Missouri) was that you could not use a faulty speedometer as a defense, because you are obligated to keep all your vehicle’s equipment in working order. Just as you shouldn’t try to use a burned out headlight as a defense for not seeing the pedestrian you ran over.
And while the officer was very nice about the whole thing (in fact, it was he who suggested I have my speedometer checked) he also pointed out that a) according to his radar gun I was doing faster than 10% above the speed limit, or else he might have given me some wiggle room and b) I was passing not just the slower cars on the highway, but the ones doing the speed limit as well, so I shouldn’t argue that I was simply pacing the traffic.
All in all, the lesson cost me almost $200, counting the cost of having the speedometer repaired,
I got s speeding ticket about a month ago. My speedometer said I was going 80. When the cop pulled me over he said that his speed while following me was 110. 30 mph is too big of difference as far as I’m concerned.
Unless radar works very differently than I understand, any of these errors caused by you not headed directly towards the gun will work in your favor, rather than against you. Doppler-shift techniques will register only the component of your motion directly towards the radar gun.
If you’re doing 50 mph, moving directly at a properly calibrated radar gun, it will read 50 mph.
If you’re doing 50 mph at an angle of 5 degrees off of directly towards the gun, the gun should read 49.8 mph (50 mph *cos(5 deg)).
That’s not much of a difference, but my point is this: this kind of “geometrical error” can ONLY result in a registered speed of lower than what you’re actually doing. It’s a cosine error.
I’m not saying that there are no other sources of error - just that this one won’t explain it the way I understand your presentation above.
If I’m missing something, I welcome correction.
Cite? No problemo.
::five minutes later::
Crap! I was going to link to articles from the National Motorist’s Association dealing with Sine Wave errors, but the site appears to be not responding.
So I have cites. But I can’t get to them.
–Tim
By Ptahlis:
not all mile markers are precisely one mile apart.
We have a five mile calibrated speedometer check run on our highway; 1 mile at 75 mph equals 48 seconds.
You can check your speedometer at any speed over a measured mile; Just take 3600 and divide it by the time elapsed; this will give you your speed, regardless of what the speedometer reads.
Conversely, if you want to know how long it will take to travel a mile at a given speed, just divide 3600 by that figure.
For example: 1 mile passed at 55 mph will take 65.45 seconds.
If you pass a mile in 60 seconds, you are traveling 60 mph.
The speedometer in my rabbit is off at the higher speeds, although it is accurate up to about 45mph.
Therefore, to travel at 75 mph, I drive an indicated 68 mph.
The site seems to be working now, and I think I found the section you were going to link to. Please correct me if I’ve assumed wrongly.
http://www.motorists.org/issues/tickets/build_a_case.html
They have an interesting take on cosine error, and it’s not one that I’d considered.
The argument is basically like this:
1 - It requires 1 second for the officer to pick you up, and another 3 seconds for the radar to “lock in” on you. Thus, the speed indicated will be that you registered no less than four seconds after you came into the officer’s view.
2 - In many situations, four seconds is enough to bring you pretty close to the officer. Close enough that your path is at a considerable angle to the sightline between you and the officer - a “high angle” radar shot.
3 - This will, they agree, cause the registered speed on the gun to be considerably less than what you’re actually doing (all other error sources ignored). Like, if the angle is 45 degrees or more, MUCH lower.
4 - You then argue that, based on the cosine error that must have been present, if the gun registered 39 mph, then you would have had to be going 150 mph! (I’m stealing the numbers from their example.) And, since the officer stated that he estimated your speed to be around 40 mph before he “gunned” you, there’s a problem here.
5 - The officer didn’t think you were doing anywhere near 150, but that’s what would have produced the 39 mph reading based on the geometry. Ergo, you’re not sure what did produce that reading, but it sure as hell wasn’t your car.
Maybe. I’m not in any position to evaluate the claim that it takes 4 seconds to get a reliable radar speed estimate. I’m certainly not willing to take these guys’ word for it. Nor would I accept a policeman’s estimate on the time required, either. Both sources are quite biased, and I’m sure both can find studies to back up their claims.
I believe that there are probably quite a number of erroneously given speeding tickets, for these and other reasons attributable to honest errors on the part of a police officer.
I believe that there are a far smaller (but nonzero) number of speeding tickets given out by officers who know or suspect that their basis for doing so is flawed, but who do so anyway.
Both types of tickets should be overturned, and I believe that the burden of proof should lie with the police.
I also believe that a number of people have given the process of contesting traffic tickets a bad name, through their attempts to manipulate the system and have even valid tickets overturned.