M5 Expert (>4000)
Join Date: Aug 2002
Location: Raleigh, NC
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In terms of outright acceleration performance, there will be essentially no gain, and depending on the speed increment evaluated, it will either be a slight loss (or significant loss if a now needed shift is in the middle of the increment) or a slight gain. When people use butt dynos, they feel the added torque to the wheels in some given gear (like 2nd) but fail to recognize the now reduced torque to the wheels once they shift to 3rd while the stock car would still be nailing the upper end of 2nd hard. It's a give and take trade-off. I did many sim runs 10+ years ago on the board with different rear end ratios, and they ended up correlating well with real world experience of some who measured before and after data. It's buried deep in the board here someplace, but the xcel and cartest images might not still be available.
You really only need to fool with changing the rear gear if you do other engine work that moves the sweet spot of the rpm range higher up the rev scale -- say porting, cams, ported throttle bodies, headers, etc, and a raised redline. At that point it likely makes sense to alter the rear gear. However, if actual, measurable gains aren't the objective, the "feel" of the shorter rear gear is of course interesting.
2019 M2 Competition 6MT LBB, slicktop
2011.5 M3 sedan 6MT Silverstone, slicktop: Dinan front swaybar, Eibach rear swaybar, pins-out-max-camber-up-front, Dinan exhaust, Michelin PS4S, Apex EC-7 9.5/10.5x19
2007 328i wagon Silver/gray: Eibach 28mm front swaybar, E93 M3 rear swaybar, 219M M3 wheels, Michelin PSAS3+
1975 CanAm 125MX2: Stock, original owner