Yes this is true. I was guessing he may be running a stock engine. At least for O.S. Thunder Tiger, and HPI engines comes with an approximate 3 and a quarter inch round fly wheel made of steel. I was just telling him he can get a smaller fly wheel like a 2 inch round made of alluminum. And yes they make em that small. And I can guarentee he'll get a 600rpm difference. It happened for me like 750rpm, but I also got a dual tune pipe. Also about that 2-speed trany for that NT. Look up rc4less.com The best prices and has everything you'll ever need.