Over the two laps that you make a pit stop on the Indy 500, you will lose about 100s, assuming you fill it full of gas from near empty. I paid attention to this a couple times while doing it in A-Spec: The in lap adds about 1:20 (80s) and the outlap adds about 18-20s. A two lap advantage will guarantee you remain in position, but realize that if you're on the same strategy as the other A.I.s, you'll cycle back into the lead eventually, if not just outright drive back into it, since they'll have to pit eventually.
Just as data backing this up, I would do the 500 in my FGT. Average lap would be somewhere around 38.5 to 39.5, depending on how good the oil was in the car. Getting the car to near empty, the in lap would usually have me cross the line at about 2:00 (as your stall is about 50 feet in front of the line). The Outlap would then have a lap time around 57s. 78s for 2 laps at speed, vs 177s for 2 laps in/out is about a 100s loss. So you need about a 2-1/2 lap lead to maintain position.
It may be because the laps are so short that I notice it but I will be 1-2 laps ahead of 2nd place and come out in 4th.
