Little help with a mission?

I am stuck on stage 8 number 6 the speed way 15 min (21 laps) race. How the hell do you catch 1st place iv came 3rd best iv done so far trying to gold them all .Maybe im pitting to early or to late i pit on lap 10 they seem to pit lap 11 im driving the viper gr3 on the straights they seem quicker than me even tho im in a faster car any tips?