Measured in liters mate, and you're absolutely doing alot wrong. 30 rps is ridiculous, you should be aiming for 7.5. Gearboxes are not complicated in the slightest sense of the word, there's no reason you can't 100% understand their operation after messing around with them yourself for 5 minutes.
But if you're against the idea of putting in effort to learn about the components yourself, I made a short guide on them a while back. https://www.reddit.com/r/Stormworks/comments/du3n8b/a_simple_guide_to_gearboxes/?utm_medium=android_app&utm_source=share
You're much better off gearing it so that your engine under full throttle is running that 7.5 rps. Cooling will be much easier, you won't need as many radiators AND your fuel will last more than 20x as long. So you could even significantly decrease the size of your fuel tank and make the plane quite a bit lighter.
Hope this helped.