I need to change out my crappy peg leg 2.73, but want to know what difference I cant expect if I change to either a posi 3.08 or a posi 3.42. In other words lets say theoretically that I run a 15 sec. 1/4 mile with the 2.73. With everything else staying the same what difference would a person see with a posi 3.08 or a 3.42 posi. Would it be VERY noticeable difference with a 3.73?? Im almost wondering if there is a calculation to this.

 

3.08 is very little change from 2.73.

3.42 would be a pretty huge change. 3.73 is even huger.

Hard to say if there's any kind of calculation for "noticeable". However, just to show how much change it is, there IS a calculation for that. 2.73 is 15 on the ring and 41 on the pinion; and 3.42 is 12 on the ring and 41 on the pinion. (no you can't just change the pinion) So your RPMs at any given speed would increase by exactly 20% ( (15-12)/15 ). 3.73 is 11 and 41; so your engine RPMs would increase by 26.7% ( (15-11)/15 ).

I think I would notice that; others may not.

3.08 is 40/13; only about a 12% diference. Even fewer people would likely notice that. IMO not worth the effort and money to be bothered with.

As far as ET, there's no way whatsoever to predict that. Depending on what the engine's ideal RPM band for power is, you can slow a car down by putting more gear in it. However, for most typical street cars and stock engines, best ¼ mile ET is usually with something between 3.23 and 3.73 gears. TPI cars, since their RPM band is so low, are VERY EASY to over-gear.