I can provide you with some general information based on the common practices in Britain and the United States up to that point:
Britain (United Kingdom): In the United Kingdom, the general rule is that you can be penalized for exceeding the speed limit by even a small amount. The police can issue a fixed penalty notice or summons for prosecution if you exceed the speed limit by any amount. However, it is commonly accepted that there is a discretionary buffer of 10% plus 2 mph. For example, in a 30 mph zone, you might not expect to be penalized until you exceed 35 mph. Exceeding the speed limit significantly or in inappropriate circumstances may result in more severe penalties.
United States: In the United States, the tolerance for exceeding the speed limit can vary by state and even by jurisdiction within a state. Generally, law enforcement officers have some discretion in determining when to pull over a driver for speeding. However, exceeding the speed limit by any amount can technically result in a citation. In practice, many officers may allow a slight leeway, often around 5 mph over the posted limit, before considering pulling over a driver. It's important to remember that this can vary, and officers have the authority to enforce the speed limit more strictly if they choose to do so.
Keep in mind that these guidelines are not definitive and can differ depending on various factors. It's always best to adhere to the posted speed limits and drive safely and responsibly. If you have specific concerns about speeding or traffic laws in a particular location, it's advisable to consult local regulations or seek advice from relevant authorities.