I would love a mod that makes trade routes faster without extending their range. the exact number is on the wiki somewhere. the 5/8 is the turn number your trader needs for one trip - the trade route will finish after a certain number of trips. Note: yes, this applies to the Incan unique Qhapac Nan mountain tunnel, so they can get some nice bonuses very early. in Civ 6, there is no timer for when trade routes end (which is just bad design). This means that if your trade route is ~9 tiles or so, traversing a single mountain tunnel will give a land route the full bonus.
Going through a mountain tunnel adds +15 efficiency score. So upgrading your roads to railroads in the industrial era with Military engineers can boost both your unit movement and your trade routes - but only the gold on international routes. This means that having more water tiles in your route increases the ratio! The +100% bonus point comes at an efficiency_score:route_length ratio of ~ 1.6:1.
Water tiles by default have a score of 2. Since each tile adds 1 length to the trade route, you will never beat the minimum ratio of 1:1. Land tiles by default all have a score of 1. That ratio determines the bonus, which ranges from 0 to +100% of the base gold value. The way it works is, the different tiles and features along the trade all have a number of efficiency points associated with them the total points for the route is divided by the length of the route itself. When you make a trade route, there is going to be a "base value" (essentially what comes from districts) and then other bonuses (from cards, great people effects, wonders, etc.) For international routes only, there is what's called a trade route efficiency mechanic that can increase the base gold value of the route. Click to expand.Because the OP just made his first post and sounds like he's relatively new to civ6, I will clarify this statement.