What Half Shaft and Tires to by

 Hello everybody I got two Questions,
1. what is the difference between a 2.5" Half shaft and a 3"Half shaft.
   Iam thinking that 3" is heavyer,than the 2.5". what happen was
   i bent the flange one Half shaft,when I was  installing the universal Joint.
  So I orded a new half Shaf that came complete.that was a month ago.
  Yesterday I attempt to install the other  Universal Joint into the other
  Half Shaf and bent the other flange.So now I must order the other Half Shaft Complete.
 Problem is when I orded the first Half Shaft, which is now installed on the car,
 When I went online to Order it,there were two Sizes 2.5"and 3".I dont want one Size
 on one side and another Size on the other. Iam thinking i need to measure the diameter
 of the Shalf thats already installed.
2. what Size tires should I buy, that will match the setup that i now have.
   I installed the Offset Trailing arms, on the Rear.From what i understand The
  Offset Arms will allow me to run up to 10" tires on the Rear.I dont think 10"
  Tires will clear the fendrer wells on the front. Iam thinking that I need a Set
  of Tires that are 10"for the Rear and 8" for the Front.
  I have the original Rally wheels.Will the stock Rally wheels take a 10" tire?
  Can I run something like Goodyear Eagle GT;s that is 10" on the Rear and
  8" on the front,? I be thankful for any input.
 

The usual 10" versus 8" discussions on here are about the wheels, not the tires.  The offset trailing arms will allow you to run 10" wide wheels with 5" backspacing on the rear without grinding on the trailing arm.  Original tires were GR70-15, which would be a P225 70/15.  The 225 refers to section width, or "fatness" of the tire when mounted on the correct size wheel.  225mm is 8.85 inches, or about a half inch "bulged out" from the edge of the rim.  If you went with  10" wide wheels, and kept the same "bulge", you would have 10.85 inches section width, or 275mm.

The next number in the tire size is sidewall aspect ratio, or the distance from rim to tread, compared to the section width.  Since you want to keep the diameter of the tire the same (so your speedo works right), with 15" diameter rims, you would need an aspect ratio of 55 to get close:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
225/70-15 6.2in 13.7in 27.4in 86.1in 736 0.0%
275/55-15 6.0in 13.5in 26.9in 84.5in 749 -1.8%

So...    If you go with 10" wide 15" diameter rims on the rear, you should probably use something like a P275/55R15 tire on the rear.

bill.

The only thing I have to say is to make sure that you have alittle saved clearance so as not to accidently rub the fiberglass.  Not just tire clearance but depending on how much your suspension will move/give-how stiff your set-up is. (if you beat it)
   I'm sure that some of these guys on here have experimented with different tire sizes on their 'Vettes.  I've only set up cars/suspension for drags in the past and it was always to get the tires as close as possible to the fender (with the tire up in the wheel well). But the rear end was set up super stiff and the front end sloopy where the weight would shift quickly for traction.

bill, Will the tire size that you recommend guarantee those tires not to rub at all?  Or will the suspension stiffness have any bearing on this?  Sounds like a pretty good size tire.

1. I'd match the half shafts.  I think the 3" ones came with automatics but not sure.  If you're bending the flanges, are you supporting them when you press the old one out?
 
2. I ran 255/60-15 on stock Rally 15X8 rims and had about 3/8" clearance to the parking brake cable bracket on the trailing arms.
10" is about max for a 8" wheel.

If the only thing you did was bend the flange just put in in a press and straighten it out again. Doesn't take that much pressure to bend it. I had two different size shafts on my car when I got it and just went with the 3". I figure bigger is better.

I won't guarantee anything.    All I can say for sure is that a P275 tire will be 275mm (10.825 inches) at the fattest point.  There are some folks on here using 10" wheels with 5" backspacing...  If you look at how your P225 tires fit, and you have 50mm extra space (25mm inside, 25mm outside), then you should be fine.

Looking at this another way, say you have standard 8' rims now with 4" backspacing with P225's on them.  If you go to 10" rims with 5" backspacing, you will need 1" extra from the rotors in, and 1" from the rotors out.  Since the center of the wheel hasn't changed (2" extra on rim width, 1" extra backspacing), you can also split the extra "aspect" of the tire (fatness).  You will need 25mm inside and 25mm outside (about an inch).

Definitely make sure that you will have the clearance, both sitting and with a compressed suspension (bumps and hard cornering).  Remember that some bodies are fitted "tighter" than others, the fenders can be 1/4" or more closer or further away from the frame on any two cars. 

Personally, I'm running 15x8 Torq Thrust II's with p225/70r15.  Haven't gotten around to putting new wheels on yet.  Hell, I still haven't gotten it back from body and paint yet!!

bill.