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Here is a useful formula for calculating the outside diameter of an automobile tire. Once calculated you can easily compare OEM to an aftermarket product. First you need three pieces of information; tire width in millimeters, aspect ratio and size. The formula goes: ((width x aspect ratio) x 2 / 25.4) + size). Below are the calculations for the TR's OEM tires and a typical modern tire.
OEM 185/80/15 (185 x .80)x 2/25.4) + 15 = 26.65 inches
NEW 205/70/15 (205 x .70)x 2/25.4) + 15 = 26.30 inches
Now with just a little math you can see the modern tire's outside diameter is 1.31% smaller than the OEM tire. This is useful in determining fit within the wheel well and impact upon speedometer reading. Not much use in determining if a wider tire will rub the inside of the wheel well.
4x4.5" bolt pattern, 0 offset for the stock rims (aka 3" backspacing on a 6" rim). Datsun Z-car (260Z/280Z & ZX/some 240Z's) wheels will bolt right on. So should Saab 900's I think (since a Saab space-saver spare can be used on a TVR 2500M).
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