What is the optimal wheel and tire size for a '69 XR7?
Here’s a great place to start. Optimal is a bit subjective but I take it to mean - Upgraded, modern, fits while looking close to period correct.
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If you search this website, there was a chart with known sizes which work and sizes which have created a problem. It has been several years since I saw the chart.
Steve
Having played with a bunch of sizes over the years I’v sort of settled on the following. If you want a good comfortable ride and light steering then 15 x 7 is a good size. For improved handling but with reduced ride then 17 x 8. Ride with the 17 x 8 can be improved with progressive rate springs and good oil based shocks like Konis.
What is your criteria for optimum? Cost, traction, steering response, classic appearance, modern appearance, other? Not enough input from you to determine what is optimum.
The answer to the original question is “It depends…” Style and cost are the primary factors when selecting a set of replacement tires/wheels. If your priority is latest technology performance rubber, your choices tend to be found at 17" diameter and larger sizes. When considering 17"x 8" wheels, your optimal backspacing on these cars is generally 4.75" to minimize tire scrubbing the fenders. Not all wheels are available with that backspacing. Tire sizes also play a role in fittment. 235/40x17 tend to be a common recommendation for the front with 245/40x17 for the rear. Larger sizes may fit but they can lead to even tighter clearances. Depending on fender clearance and suspension choices, occasionally narrower tires with slightly taller profiles may fit more easily. For example 225/50x17 front and 235/50x17 rear may provide clearance plus a slightly more supple ride compared to the lower profile tires. It usually comes down to personal preference and how much tire scraping is/is not acceptable.
Tires in the 14" 15" and 16" diameters have been left behind when it comes to performance and the common BF Goordrich Radial T/A has been around for decades with little to no perceptable improvement in performance.
I suggest attending local car shows and Cars & Coffee events and talk to owners of those cars running on wheels that appeal to you and ask about them. Not necessary to reinvent the wheel on this ! Good luck with your search.
I run 245/45-17 up front and 275/40-17 rear but could easily fit 295’s in the rear if the tires I use were available in that size. These are on 17x8 and 17x9.5” wheels respectively.
As noted above, tire selection in 14” and 15” is slim these days. Depending on what you want to do with your car-cruising/ice cream, autocross/track days, etc-that may or may not be a problem.