You may already know this / it may be obvious, but I found this out in an odd way because I watched a tutorial from pre-gearbox ratio reversal and ended up finding the opposite in-game. So, the old ratios were things like 1:3, meaning if your engine was rotating at, say, 7 rps, then you had 21 rps at the other end of the gearbox. However, now it's 3:1, so you actually have to reverse your gearbox's orientation of attachment to primary power to get 1:3.

In many cases, much more than 1:1 ratio results in insane speeds which lead to pitching up, catching air, and general carnage. Gearing down is also probably really good for tugging applications.

Since 7 engine rps is reputed to be a good sweet spot for engine efficiency, perhaps you could simply adjust your gear ratio such that your engine is turning at 7 rps when you're at a "nominal" cruising speed. But take that with a grain of salt, I'm def. not a sme on this stuff.