Thats a pretty good rule of thumb, but 21/78 in an off road car with a 7700Kv motor seems extremely high to me. I know my T4 has taller tires, and possibly a different transmission ratio, but when I tried a 7700Kv in it I had to use 12/87 to keep everything cool. It was still way too fast for an offroad track with that gearing. I do agree about getting a temp gun so you know exactly what temps youa re hitting. A few buddies and I always disagree how hot an ESC/motor feels before actually measuring it. But typically around 150F will burn my finger after a few seconds, higher than that and I can only touch it for one second or less.