Let's say that I drive down an interstate highway at a speed of 68 miles per hour, where the speed limit is 65. I think that I am going 63 miles per hour because my speedometer is off by five miles per hour - without my knowledge.
Would I be responsible for speeding even though I have no reasonable way of knowing that I am breaking the speed limit?
Note: I would like to assure everyone that this is entirely hypothetical. I have never encountered anything remotely like this, nor do I expect to.
Yes, you would be responsible. Maintaining the vehicle in a state that enables compliance with the law is the owner's responsibility, and it is a driver's responsibility to comply with the speed limit. There is no knowledge or intent requirement in a speeding violation. That said, a judge might show leniency if you came to court with documentation of a repair or recalibration of the speedometer after the citation.
This varies widely depending on the jurisdiction, but you'll often see speeding tickets downgraded to a non-moving offense like "improper equipment" when the speedometer is broken.
You'd want to check the jurisdictions you're interested in to see how they define improper equipment.
Edit: here's an example from NC that sig_seg_v alludes to:
N.C.G.S. § 20-123.2 Speedometer.
(a) Every self-propelled motor vehicle when operated on the highway shall be equipped with a speedometer which shall be maintained in good working order.
(b) Any person violating this section shall have committed an infraction and may be ordered to pay a penalty of not more than twenty-five dollars ($25.00). No drivers license points, insurance points or premium surcharge shall be assessed on or imputed to any party on account of a violation of this section. (1989 (Reg. Sess., 1990), c. 822, s. 2.)
In NC, DAs frequently allow you to enter a "responsible" plea for the improper equipment rather than a "guilty" one for speeding. This allows you to avoid points and/or insurance ramifications.
Plea bargains can be beneficial to the jurisdiction for a few reasons: 1) they're specifically authorized by statute (N.C.G.S. §20-141 (o)), 2) prosecutors have high case loads and negotiating plea bargains decreases that volume, and 3) the county with jurisdiction may collect surcharge revenue from the plea (N.C.G.S. §7A-304 (a) (4b)).
It absolutely depends on jurisdiction, to my knowledge these laws are usually (always?) state-level. In my state, North Carolina, an "exact speed limit" state, for years the law was that if you were clocked less than 10mph over or under the speed limit, you were ostensibly traveling at exactly the legal limit, because police radar could legally only measure within +/- 10mph of the actual speed at which a vehicle was moving.
(This was, historically, due to federal legislation that mandated a maximum highway speed limit of 55mph in order for a state to secure federal highway funding. The state legislature complied with these regulations without compromising their constituents' ability to drive at 65mph by posting a 55mph limit and allowing motorists to travel 10mph over the posted limit. Those federal restrictions have since been lifted.)
Recently, a new party was voted into control of the state legislature, and the law was changed -- police radar is now legally exact to within 1mph, and a driver is in violation if they don't travel the exact speed limit on a given roadway. In practice, however, motorists found in violation can often plead guilty to a lesser charge -- that is, to a charge of "improper equipment" -- so the prosecution can avoid the costs they would otherwise incur by actually having to prove their case. This doesn't apply to severe violations, like going 90mph on a freeway marked 65mph or 55mph in a 35mph zone, which are often criminal misdemeanors, not traffic citations. In general, whether someone is eligible for such a reduced charge depends on their history of traffic citations and on the local district attorney's office and the presiding judge.
Some other states do not have an "exact speed limit" and drivers are responsible for maintaining a posted minimum speed (where applicable) and not exceeding a posted maximum speed. In these states, minimum speeds are often present on interstates and other major divided highways and often absent elsewhere. Where in doubt, follow the speed of traffic or stay 5mph under the posted speed limit, as indicated by your perhaps glitchy speedometer.
To answer your question explicitly, and in general, yes, as an auto registrant you are responsible for keeping your automobile in good working condition. However, there are often charges less costly than exceeding a posted speed limit for which you can accept a plea bargain or for which you can provide evidence to the court to reduce your responsibility.
(My source is personal knowledge and experience as a licensed driver in NC)
This question is no longer one of law, but rather of "justice."
As far as the law is concerned, you are still in violation. As a practical matter, "justice" says you will not be chased for a 3mph violation.
If it does happen, the principle is extended further. Your defense is to take the matter to traffic court. A sympathetic judge may "buy" your story and let you off or plead guilty to a non-moving violation. There may be other things working in your favor; the officer may not have filled out the ticket properly, or whatever.
I have beaten all but one of my traffic tickets by going to court. In one case, the judge couldn't read the officer's handwriting, in another case, the officer alleged that I was parking in a no parking zone on a "main" street, but gave the "incorrect" cross street (one where parking was allowed)>
I guess that in most countries the driver is responsible of the road worthiness of the vehicle, including speedometer. otherwise everybody would be speeding with broken speedometers. While traveling in Senegal, Africa, I didn't see a single car with working speedometer. Mayby that's valid excuse there.
Remember that on most cars the speedometer shows too much speed to be on the safe side, so the drivers obeying the limits drive too slow. I personally use gps or smart phone speedo app to measure the accurate speed, and set my cruise control to speed just below the maximum allowed over speed of 10 km/h.
Of course, it is your responsibility for what happens to the car, it is impossible that you do not realize the broken speedometer, another question is whether the speedometer does not score regularly, but the blame is always yours.
