No you've got yourself backwards there. The drive shaft speed is directly proportional to the wheel speed as the ratio is fixed (3.42 or what every yours is). The only ratio that changes is in your transmission. Therefore, both your wheel speed and drive shaft speed will be faster in 3rd than in 2nd at a given RPM. Your drive shaft speed will be the exact same in 3rd gear no matter what gear ratio or tire size you have.

 

I dyno in 3rd I got 4:10's with a 28 inch tall tire ...on a stock shaft no issues yet

 

Trans gear will change the shaft speed, but in 3rd gear at 6000rpm, your shaft is going just as fast with 3.23's as it would be in 3rd at 6k with 5.13's.

 

I think we did say the same thing. Since the only ratio for a given set up that changes is the trans gears then the drive shaft is proportional to the trans gearing. The rear gear and wheel are driven off of the trans gear. Wheel speed doesn't have anything to do with drive shaft speed except that it is what is driving it. Wheel speed is determined by rear gear ratio and tire diameter along with trans gear. So the drive shaft is spinning faster at a given rpm in 2nd than it is in third? Or is it that to maintain the same mph the Drive shaft spins at different speeds? i.e. to maintain 60 in 4th the DS only spins at, lets say, half the speed it would have to in 2nd to maintain the same speed? I am probably making this way to over complicated. So what is your guyses opinions on my original post? 2nd or 3rd?

 

I've always dyno'd in 3rd gear. If you have factory tires, you should be wary because they are not rated for that type of speed and the sidewalls will distort. Didn't seem to hurt my tires any, but with the speed rated BFGs I have there was no problem.

 

i dyno'd mine in 2nd, 3.42 gears with 32" tire 4x4 i was like 111mph at the top of 2nd lol
You always should try to dyno in 3rd or 1.00:1 ratio because not only is 2nd aprox 5% lower numbers but your tune will be off a little because 3rd gear will apply more load on the engine than 2nd
If you have a big one piece aluminum shaft do not try it lol!

 

I dyno'ed mine in 3rd with 20in wheels an stock 275/55/20 tires which is close to a 31 an went 150+ at 6200 an pulled fine but I was steadily walking away just in case lol.. but I do have a steel shaft from factory