unfortunately, there is no consistent speed. some people get up to 130 on a regular basis w/o a chip while others cannot pass the 108 mark. myself, I've gotten to about 110 before having to let off the gas due to needing to get off the freeway.
the reasoning behind it, per consensus of the board, is the speed rating of your tires. hypothetically... if your tires are limited to 95mph, but your car can go up to 150mph, then you run the chance of killing yourself and others if you try to "bury the needle".