I'm going to assume you're thinking staightline racing -
Well, if you go too short in the rear, with a lot of power, you could be just wasting a lot of time spinning your tires unless you have a great set of meats and your suspension is dialed-in. Off the line, unless you can get good enough weight transfer, all that gear is going to be more harmful than good. I know there are gear calculators out there, so it'd be nice to see what tranny gear would be optimal to match the rear gear to, so depending on if you're running an 1/8th mile, or the 1320, you could set your car up to be in the powerband as you go through the line. I think GMHTP was running their stock G8 GT through the traps just after shifting into 4th gear, so maybe 4.30s would get you close. I think, depending on your tranny, and subsequent engine, you're going to have to consider redline and fuel shut-off too because it won't do any good to expect to run your L99 to 6500 RPMs when it redlines at 6000 (I don't recall if that's the correct redline..) More-or-less, you should probably put together a plan on what you plan on running, power-wise, get an idea of your powerband, and then calculate the gear you'll be in going through the traps, and adjust your ring and pinion to that number.
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