So where do you begin when...
So where do you begin when measuring for new wheels and tires? A logical starting point is with the existing (presumably stock-dimension) wheels and tires. A straightedge and tape measure can help you determine backspacing.
Deflection: The measured difference between the tire's free radius and loaded radius when mounted on the measuring rim, inflated to the test pressure and placed under a prescribed load.
Free Radius: The distance from the wheel axle centerline to the outer tread surface of the unloaded, properly inflated tire.
Loaded Radius: The distance from the wheel axle centerline to the tread contact surface. Measured after the tire has been mounted on its measuring rim, inflated to the test pressure, and placed under a prescribed load.
Overall Diameter: The linear distance between the tire's tread surfaces measured at the widest point. This measurement is taken with the tire mounted on the measuring rim and no load applied.
Overall Width: The exterior measurement of a tire's width from inner sidewall to outer sidewall (including protective ribs, lettering, etc.) when properly mounted and inflated.
Section Height: The measurement of the vertical distance between the tire's bead seat and outer tread surface when properly mounted and inflated, but with no load placed on the tire.
Section Width: The measurement of a tire's width from sidewall to sidewall (excluding protective ribs or decorations) when properly mounted and inflated, but with no load placed upon the tire.
Then compare your figures...
Then compare your figures to the room available in your wheelwell. Estimate how tall your wheel and tire package will be, then measure the distance to the fender lip and any inner side obstructions (frame, suspension components, etc.). Take these measurements with the suspension compressed to approximate ride height. Make sure to take measurements at several different points on the wheel radius, and try it with the steering turned to full lock in both directions.
Tread Width: The distance from the outer edge to the inner edge of the tread.
The Metric System
We often take for grated that everyone knows how to "read" the sizing information on radial tire sidewalls. That's not necessarily the case. Here's a refresher on P-Metric sizing.
Example: P215/70R14
P = Passenger car tire
215 = Section width in millimeters
70 = Aspect ratio
R = Radial construction
14 = Rim diameter
Add It Up
"Plus-sizing" is a common concept with most enthusiasts, but is still worthy of discussion. This practice allows you to achieve a higher level of performance and handling by mounting tires with wider section widths and lower aspect ratios to rims that are larger in diameter than stock wheels. The goal is to maintain the same approximate overall tire diameter while gaining a wider tire footprint and stiffer sidewall. Just keep in mind that, from a visual standpoint, a lot of '50s and '60s cars need a relatively tall sidewall to make them look right.
*When increasing section width, rim width may have to be widened to accept a wider tire.
**Adjust according to your needs. Tires with too low an aspect ratio may look out of place on vintage cars. Try finding tires that "fill the wheelwell" of your particular car.
Rules Of Thumb
Plus 1:
* Increase section width by 10mm*
* Decrease aspect ratio by 10 points**
* Increase rim diameter by 1 inch
Plus 2:
* Increase section width by 20mm*
* Decrease aspect ratio by 20 points**
* Increase rim diameters by 2 inches