I have a 94 z28 a4. I could not for the life of me find a clear answer. What kind of aceleration difference would i have if i swapped my rear end (2:73) for a 3:73? And what would my top speed be then? What kind of 0-60 time difference would it be?

 

Acceleration depends on many factors. However, top speed can be calculated. Assuming you have stock wheel/tire size (245/50x16) and a 4l60E A4, your top speeds should be close to the values listed below.

Primary drive ratio (0 to compute) : 1.00
Seconday drive ratio (0 to compute): 3.73

Primary Ratio = 1.00
Secondary Ratio = 3.73
Final Drive Ratio = 3.73

Enter number of Transmission gears: 4
Enter Gear Ratios for 4 gears
- Gear 1 Ratio: 3.06
- Gear 2 Ratio: 1.63
- Gear 3 Ratio: 1.00
- Gear 4 Ratio: 0.70

Wheel Radius in inches (ground to axle center)
Enter Wheel Radius : 12.0
Wheel Diameter : 24.00 inches
Wheel Circumference: 6.28 feet

Enter RPM to compute gear speed in MPH: 5000
Gear Speeds at 5000 RPM
- Gear 1 Speed = 31.26
- Gear 2 Speed = 58.69
- Gear 3 Speed = 95.66
- Gear 4 Speed = 136.66

Enter RPM to compute gear speed in MPH: 5500
Gear Speeds at 5500 RPM
- Gear 1 Speed = 34.39
- Gear 2 Speed = 64.56
- Gear 3 Speed = 105.23
- Gear 4 Speed = 150.33

 

alot! speed limit is 70mph

 

Does anyone have numbers? 0-60 times before and after the swap?

 

Nobody really pays attention to 0-60 times. It's more about quarter mile times. When I was new to automotive performance, I once asked a guy how fast his (highly modified for drag racing) car did 0-60. He laughed at me and said "about thirty feet."

It's hard to say how much you'll gain, as it depends a lot on surface conditions, your skill, your tires, and your suspension. Even if your car is bone stock, you are almost certainly not using the factory-spec Goodyear tires, and I genuinely hope you've replaced the shocks by now.

Ballpark guess: I'd say you'll gain a half second in your 0-60 time, assuming you can get it to hook up.

The math posted above is highly suspect. The wheel radius is 12.8", not 12", and the fuel cutoff of an LT1 is at 5800 if I remember correctly. Regardless, your car's top speed is limited by aerodynamic drag, not gearing. This gear change will probably only increase your top speed by 1-3mph. I believe the stock spec was 155mph.

 

The key here is that the 3.73 gears will make it feel like you added 36.6% more torque at the rear wheels, at least off the line. To reach 60 MPH (with the correct tire size - I'm going to assume that with the 2.73 gears the car did not have the Z-rated tires, but rather the 235/55-16 base V8 tire = 13.08" diameter) here's what changed:

2.73 rear:

Launch in 1st gear, rev to 5,600 RPM (no benefit in revving to 5,800) and shift at 52 MPH. In 2nd gear, revs drop to 2,980 RPM.

Continue to 60 MPH in 2nd gear, reach 60 MPH at 3,440 RPM


3.73 rear:

Launch in 1st gear, rev to 5,600 RPM and shift at 38 MPH. In 2nd gear, revs drop to 2,980 RPM.

Continue to 60 MPH in 2nd gear, reach 60 MPH at 4,700 RPM