Driving in Japan (Nara prefecture) as a tourist, I noticed that while the speed limits are generally very low, at least for my German feeling, nearly everyone seems to do at least some degree of speeding. On a curvy mountain road clearly marked with a 30 sign (all units kph), people were driving between 45 and 50. On a motorway with a small 70 speed limit sign mounted high on the left side, people were driving 100 to 120 in the right lane. Is speeding widely tolerated in Japan or am I misunderstanding the speed limits?
TL;DR: Yes, it's common, but exceeding the limit by more than 10 km/h risks getting stopped and fined by the police.
Japanese speed limits are indeed very low: the kind of straight, wide country road in Hokkaido that would have a limit of 100 in Australia will be 50, and even grade-separated expressways can go as low as 80. (Then again, hair-raising twisty mountain roads will be 40 throughout when in Australia every other hairpin bend would be plastered with 20 km/h recommended speed warnings.) A theory I've heard advanced is that Japanese police have targets that strongly incentivize reducing highway fatalities, and lacking other levers they constantly and often successfully lobby to reduce limits.
What this means in practice is that many people speed. Speeding fines only start at 10 km/h over the limit, meaning that exceeding the limit by less than 10 km/h, while technically illegal, is usually ignored. At the other end, getting caught speeding over 30 km/h above the limit can result in formal charges and immediate license suspension.
So if/when people speed, they tend to do so in the 10-30 km/h over band, no more. However, speed traps are quite common and typically operated by a team where one officer hides in a bush with a speed gun and their buddies catch the offenders down the road, meaning that by the time you see the police cars, it's too late. If caught, expect hefty fines (tens of thousands of yen) and demerit points if you have a Japanese license.
In my experience, it's best to go with the flow: drive at the same speed as all the other normal drivers (not the hotheads in the right lane), slow down when they do (the regulars know where the cops lurk), and you'll be fine. As always your mileage may vary.