ok, this is a two part question : 1st, how do you tell a GM 8.5" rear from
a GM 8.6" rear, and what does all ZR2's have..

Question#2, 33" tires should i gear to 4.11 or 4.56, which would be better in the ling run..
Thanks Robert

 

4.11s vs. 4.56s... what will you be doing with it? Serious off-roading, primarily road travel, towing.. what sort of terrain will you be driving in, what sort of speeds do you intend to run at? Lot of variables to consider here. 4.56 is the lower gear ratio, so you'll run higher RPMs at speed. 4.11s with 33s is good for a vehicle which sees mostly road use, with a little 4x4 usage thrown in.

 

mostly on hiway, with some offroad tossed in, no hard core stuff, i drive no faster than 65mph on the interstate, mostly to conserve gas

 

If I were in your shoes, I'd probably go with 4.11s. With the 1:2.72 ratio of the NP231 or NP233 transfer case (depending on whether you have a manual or auto), it should be good for a 'general purpose' vehicle.
A good way to figure out what RPMs you'd be running at highway speeds is this formula:
A/B = C

A would be (Engine RPMs x 60)
B would be (Axle ratio x tire revolutions per mile x transmission ratio)
C would be your road speed at whatever RPMs you're running

To determine how many revolutions per mile, you take the tire measurement (in this case, 33 inches) and multiply it by 3.1415. So you get 103.6695. This gives you your tire circumference.

Then you divide the number of inches in a mile (63,360) by your tire circumference to get your revolutions per mile (in this case, we'll round it to the nearest number, which is 611).

So you have 611 revolutions per mile on those 33s.

The 4L60-E transmission (assuming yours is an automatic) has a .70 overdrive ratio.

So, let's see where 2000 RPMs puts you with 4.11s....

(2000 x 60)/(4.11 x 611 x .7) = 68.265....

So you'd be running about 68 at 2000 RPMs in fourth gear.

With 4.56s...

(2000 x 60)/(4.56 x 611 x .7) = 61.528...

So you'd be looking at approximately 61/62 MPH at 2000 RPMs.

You can fiddle around with the numbers as you wish, and figure out your RPMs at what speed you're running at.

 

after doing the math, i will go thit the 4.11, depending on the manufacturer, possibly 4.10..

So, how does a person tell a 8.5" rear from a 8.6" rear??

 

I'd do 4.56 gears.

 

You could also consider 4.22s, 4.33s, or 4.44s, if you're looking for a medium between the two. I had 4.56s when I was running Super Swamper 36s behind a 454 on a C/K 2500.. if I had to do it over again, I'd have probably gone with 4.33s.

 

4.10 gears would be the closest to the factory tire/gear ratio, but with the added resistance with the bigger tires and lift the 4.56 ratio works well. I have 4.88 gears with 35s(same ratio as 33s and 4.56 gears) and it works very well on and off road. I run about 2100 rpm at 65mph

 

so how does a person get an off value gear ratio, im not able to find anything in between 4.11and 4.56,

 

I didn't think you could, but I've been proven wrong many times.

edit: Proved myself wrong: http://www.summitracing.com/search/D...-Ratio/4-30-1/

and they make a 7.2" matching set for the front