I run 30% because it has the most power and apparently the more nitro you have the cooler it will run. But on the other hand I would run what your engine manufacturer suggests.

Your best bet is to stick to what the Manufacturer suggests. I.e. Look at your Engine Pamphlet or Manual.
I race, and run 20% in my Truggy and Buggy. I run an OS Speed Spec III in both.

The most common for racing is 30%

Most engines only see a 200-300rpm gain for every 5% nitro you add. Since it’s hard to determine actual engine rpm on buggy engines, you have to judge by how fast your lap times are or how fast the truck can go in a given length of straight track. The reality is you get about 500-600rpm; maybe 1,000rpm more on 30% than you get on 20% and your tuning window is a little wider. But in racing, it’s not the engine you’re running, it’s your driving. I’m willing to bet a better driver using 20% nitro can beat a good driver using 30% nitro.

I’ve used fuels containing 0% nitro up to 50% nitro. I’ve found no real good reason to use more than 20-25% nitro as there is no significant benefit beyond a touch more power.

I’d suggest - for bashing - or even racing - a fuel comprised of 20-25% nitromethane, 11-12% oil, and a medium heat range plug. Ultra hot plugs are fuel wasters. Ive found in a bashing environment in the engines I run, a 60/40 blend of castor to synthetic oil to be ideal and I use hot body medium filament plugs like the Picco P6TH and Novarossi C6TGC/CTO6. Ive used a P6TC Picco plug off road and it ran just fine, but off idle had a little lag as the plug needed to heat back up. Hot body plugs retain more heat and have less lag.

OP, this is pretty simple. The higher the nitro content the more power you will have on tap. For some this is good, for most this is bad. I see a lot of racers just getting started in the hobby and they are ready to hit the ground running . There is nothing wrong with building a first kit with all premium running gear. But for most an os speed or novarossi or a premium reds on 30% is to much power. I see racers out trying to drive and they are just all over the place because they cant control what they are driving. If they only knew to much power is a handful. Now your Jared Tebo's, Robert Battle, Ryan Maifield , you get the point. Those guys can handle that kind of power.

I have ran 20-30% and ended up settling on 25%. Fuel can be used as a tuning option just as a hop up part. I dont do this but it is useful. If I am on a big high grip wide oped track I could see my self running 30%. On the opposite end if I am on a low grip surface where speed is not a factor just being able to drive and not crash I may go to 20%. But as mentioned I run 25% as it is a good middle of the road. I hope this helps you.

1: Milage
- where there is less nitro, the volume will be filled with methanol while the oil content will stay the same. More methanol will give a better milage.

2: Tuning.
- More nitro will make an engine better run, with that a much wider window of tuning were within the engine will run OK, a low nitro content can make the tuning very critical.

3: Ignition timing
- This is what most people do not know. More nitro will shift the ignition timing to a sooner moment. If you use more nitro then advised for the engine you will get a too soon ignition causing heat, a lower top RPM and detonation with the risk of blowing plugs. So with an engine configured to 25% running on 30% could only need a colder glowplug than advised (colder plugs will shift ignition to a later moment). If te difference between advised and used nitro percentage is larger it is adviseable to add an extra head shim (lowering the compression will also shift the ignition to a later moment).

But due strickt anti terrorist laws we are alowed to run 16% by rules, and we run it w/o any issues. Somewhere next year or in 2021 new laws will forbid model fuels with more than 16% nitro.