The 275 formula is based on two things:
1) Off the rack wheels where width and backspacing availability is limited.
2) 100% stock: No modifications to wheelwell lips or any other massaging. Given those two guidelines, a 275 tire on a 9.5" wheel is the maximum "no brainer" package.
If, on the other hand, you want to get creative--and have money for custom size/backspace wheels the 285 tire will fit, and maybe you can even jam a 295 on a 69 if the car is not too low (but I don't recommend it). Custom wheel manufacturers will sell wheels in .5" width increments and .5" backspace or even .25" backspacing increments.
285mm converts to 11.22" You have a little more than that minimum clearance in stock form. The minimum clearance is right around the 10 o' clock position in the wheelwell. The project becomes way easier if you roll the wheelwell lips (Actually, it's almost mandatory). A 10"/285 combo with 5.5" to 5.75" back is fairly easy. When you're running such tight clearances, be prepared for a lot of measuring and potential do-overs. Don't rely full on others backspacing measurements as your car may vary (even from side to side). So you still have to measure.
So if you are creative and ambitious, you can stuff that extra .5" or more of rubber without minitubs. It's just way easier to either run that off-the-shelf 275 combo or get in there and hog out the area with DSE minitubs. The minitub kit takes quite a bit of of work, but obviously a whole world of options opens up.
In the case of my own 68 Camaro 285/10" with DSE 3" springs combo, I will be milling .200" off the mounting pads (offset) of my rear wheels to tweak my clearances. 'Cause I just ruined a tire on a road trip. You know: measure once, cut twice.
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Steve Chryssos
Ridetech.com
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