can i ask for some gearing help?

now:
losi xxx-sct
tekin rs blinky
reedy sonic 17.5
stock timing 20 degrees
78/23
thunder power g6 pro race 5300 65c 2s
sdrc raceway: med to high bite indoor off road clay track

i think i need to go to a smaller spur gear. my motor was toasty after a 10 min run. i was getting killed on the straight by a tekin 17.5. what fdr should i aim for and why?