Rearend and tire combo question.

[whytry]

On my 68 Camaro I have 18x8 with 4.5 B/S and I have a 255/45/18 on it and it is very close to the outer part of the fenderwell and there is adequete space on the inside to fit a 275/40/18 back there but if I leave everything alone it looks like it will rub on the outside.... Now I am getting a new rearend built and I have the option to shorten my 9". I was looking at shortening it 1/4" - 1/2" on either side to get the clearance I need. Does this sound right? From what I have read in the past my wheels should have been 4.75 B/S, not 4.5. If I shorten the rearend by 1/4" - 1/2", it would be like ordering wheels with 4.75 B/S right? Also giving me the clearance to fit the 275/40 I want. Any help appreciated...

[astroracer]

Don't just look at the wheel well here... The closest point you need to worry about is the spring pack and possibly the shock. Measure from the back side of your 255 to the spring pack (front and rear) and the shock and use the shortest dimension as your baseline. This should probably be done with the axle in full droop...
The 275 is 20mm wider then the 255 but you have to split that inside and outside so it is 10mm (0.4") per side additional.
Subtract 0.4" from your baseline dimension.
Then subtract 1.5" (for tire clearance) from that result. This will give you the distance you can shorten the rear end per side. If you want more or less clearance use a larger or smaller number.
Now measure from the outer tire face to wheel well lip. Use the inside edge of the lip at it's closest point to the tire (probably the top).
Add the previously calculated dimension to this measurement then subtract the additional 0.4" from that result. This will give you the clearance to the lip with a 275 tire... Roughly...