[0] default 60km/h[1] for undivided inter-urban roads though it's common to get a 50 or even 40km/h speed limits, the default highway speed limit is 100km/h[2] but it's commonly lower than that, and I've seen limits as low as 70km/h on highways, keep in mind that all highways are toll roads and they're expensive as hell (entry fee of ~¥200 plus ~¥25/km[3], Hokkaido has a special tourists rental ETC[4], Honshu does not)
[1] under 40mph
[2] 62mph
[3] ~¥40/mi
[4] from ¥1800/day for 2 days to ¥800/day for 2 weeks or more, unlimited travel. With regular ETC, going from Sapporo to its airport (New Chitose) is ¥1400.