US speed limits in practice
The table below gives an indication of the maximum daytime speed for cars on the fastest roads in the state. Night-time limits, limits for trucks, and limits for other roads are less. Some states which were keen to reinstate a higher limit have actually pulled back and reinstated the 55mph limit on some roads due to the very big increase in fatalities. Note that the table is only a guide, limits do change periodically, and the speeds are only for the fastest roads in the state.
On the other hand, a posted maximum speed does not give the automatic right to drive at that speed. There is a requirement to drive at a speed that is "reasonable and prudent" depending on weather and road conditions.
Speed limits in other areas will vary too, and the lowest limits you'll encounter are posted in and around schools. Here the limit could be as low as 15mph, and will be imposed according to the details posted on the signs. For instance, it might be imposed while yellow lights are flashing, or when children are present, or between certain times of the day. It might seem a bit strange to slow down to a crawl with no other cars around, but if you don't you risk a committing a very serious traffic offence.
Penalties for speeding in a school zone vary from state to state. Some have mandatory fines (up to $2000), others have jail sentences (in Arkansas, failing to observe a school speed zone can attract a 1- to 10-day jail term for a first offence, in Michigan the maximum penalty is 90 days!), others double the normal fines. And "speeding" for these penalties can be as low as 3mph over the limit!
While on the subject of "serious", within the national rules, driving 15mph or more above the posted limit is classed as a "serious traffic violation", and more than 20mph over the limit is treated the same as "reckless driving".
Another area where speed limits are stricter is in "construction zones" (road works). Lower speed limits will usually be posted, and failing to observe them can also attract significant penalties. These are normally less than for school zones, and are usually double the normal fine, but higher penalties are available.
As a visitor to the US, if you're caught speeding, then you will normally be treated no differently to a resident. However, some areas (and it varies from county to county, rather than state to state) are inclined to be more lenient with visitors. But if you do find yourself the subject of a speeding violation, then simplicity and honesty with the police are the best ways forward.
If you do attract a penalty, then the easiest way is just to pay it. If you fail to do so, one of several things will happen, depending on the area and severity of the offence.
In some cases, the car rental company will pay the fine on your behalf and recover it from your credit card. You can't complain about this, because you probably agreed to it in the rental agreement you signed.
Alternatively, the local court will "dispose of the violation" and not bother to pursue the fine. If this happens, it may remain on record ready to surface if you ever get caught again. It may also appear on your immigration record and could prevent your entry on a future occasion (but it would have to be a fairly serious offence for this to happen).
If the court doesn't dispose of the violation, and it remains unpaid, it's difficult to predict what will happen. In the worst case, you could be arrested at immigration on your next visit and be put in jail.
And not really related to speed as such, the states have a series of "aggressive driving" laws. As an example, in Delaware, if you commit three out of a list of about a dozen offences (including overtaking on the right, failing to signal when turning and driving too close to the car in front) you are guilty of "aggressive driving", and the minimum penalty is 10 days in jail on first offence!
Finally, if this all sounds draconian, it isn't really. Drive carefully and sensibly, as many Americans do anyway, and you'll not have a problem.