You didn't say the year of your car. This could make a difference in the size of the tire. The 65-67 may not be able to run a larger tire. Just be sure to test the size first. I have a 66 that can not run anything bigger than a 185/65/15 or it will rub against the front inter wall of the fender. It is also only about 4mm from rubbing on the struts. In the rear it is only about 5mm from the tire and the inter lip of the fender. To get more room you can roll the inter lip flat of both the front and rear fenders. It will also depend on the profile of the tire some tires are flat sided and others will be more rounded. The lower the height of the tire will also make a difference.
The 68 even though it is still a SWB flat side. It came with a spacer for the rims and may be able to run a larger tire.
The 69 and 912E's have a longer wheel base and also have flairs to accommodate larger tires. Though a lot of folks do get away with a larger tire. My car can not.
