a tuned 67 with a 12 oz tank should easily last for a 6 lap plus race. i used to race 67 and 90 class 25% maximum nitro and all was good [ mpg ] using 12 oz tanks, if your not racing you don,t need any nitro in the fuel mix but if you want you can pop in 10% for better starts etc. keep the 65% for the 21,s and lower.