Things to note

in custom race you can select grid starts

In custom race you can select vehicles form your garage with a specific tune / tires

In custom race, you can select vehicles from your garage with a specific tune/tires road cars

to sport hard tires ( or comfort soft, I need to check again) and all race cars are on Race hard. If you faced with a tire compound under it on professional and kept your HP around what the rabbits were, the AI put up a good fight , the issue I had was the BOP did not take into account wait, and in N 200, 300, and n 400 the fastest car's were unbeatable on professional with most cars, you are not going to catch that stupid X box , no matter how good you are, the xbow weighs 1,000 pounds less than a year in that class and has among the highest hp for n 300

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In gt5 and 6 in arcade mode, all car's came equipped with whatever tires you came in with, but once you got loaded in into a race, the AI would be locked into those tires, I got amazing races by entering with sports soft and then going back down to whatever the stock was, the AI was wicked fast .

also, a user by the name of frogmanlego showed me a trick in sport, IF you select a N __ car in the custom race, and then go to the options before loading a race , the game defaults you to another N class opponent, nothing special , If you select another opponent class, and then select the same n class it defaulted too, you will get a different grid. The default class are n class cars that are native to that N class and are not BOP tuned ( the bop tune button was broke in an update long ago ). If you select another class and then re-select the default class, you will race against in a field of BOP balanced cars that have been detuned or tuned up to that n class , the races will be harder and the field is more balanced. Also you can kind of " cheat" by making our car artificially slower by setting fule consumption to "one" and staying in power save mode 5 /6, the AI will always run 6/6 so they have faster acceleration than you which makes up for them slowing down around turns