If you have an automatic transmission and you are stopped and about to race someone in a drag, what gear should you start off in? Overdrive, drive, 2, or 1? Also just please explain why that's the case, thanks in advance

With a 'street' car I would say most would start in 1 in order to hold low gear a bit longer, putting you more in the 'fat' part of 2nd gear when it shifts. By 'street' I mean a vehicle with moderate modifications, if any.

For a dedicated drag car (one that is much quicker than street cars), the auto would shift faster in D or OD than being shifted manually, plus commonplace trans mods at this level only increase the performance of an auto.

When a particular vehicle falls in the grey area betwen the 2, only experimentation will reveal the best method.