This won't help you much there, but why on earth do you want to change the gas to A button, which is digital. Ie. it's on full or then it's off, which is a bad idea when having to go with exact speed in curves, exiting steep corners etc. Having analog shoulder triggers was a stroke of genius from Xbox developers, for the first time car games are somewhat drivable without steering wheel.
And don't say that precise speed adjusting can be done with gears, since it can't be