Overall tire diameter increase pros and cons?

Hi,

As I'm looking into tire choice for 2022 season, I've run into a problem. I have a Mustang Time Trial/ Hill Climb car (3500lbs, 600+whp) going from semi-slick to slick tires. I used to run 285/30(35)R19 tires (~650-680mm diameter), and since I'm moving up a class, I can now run wider tires.

Ideally, I want a 325 width tire with similar overall diameter, but the only size available from my sponsor is a 325/710R18 (312 tread width), which has a diameter of 708 - about the same as factory Mustang tires.

I am not too worried about the gearing or aero affected by the diameter increase as I can make adjustments to those, but I am concerned about the extra unsprung rotational weight, as well as how the much taller side walls would affect the handling characteristics.

Alternatively, I could go down a size and put on 300/680R18 (304 tread width) or 300/650R18 (300 tread width), and their threads are about (only) 8-12mm narrower.

Any input is welcome. Thanks in advance.

Hello, Tire rollover would be my biggest concern as it would affect the handling dramatically, i would suggest testing even if it meant having to buy tires,

As Ross said, the biggest concern may be the change in the tyre construction and characteristics - these can be VERY different between brands and their product lines, even at the same pressure.

With the unsprung mass and moment of inertia (flywheel affect) you will need to check for yourself what the maufacturer data states. Remember, because of construction and design, you may increase one and decrease the other, or vice versa. If you can afford it, get the lightest wheels that have the strength you need, because not only are the previous items affected, but you're going to be affecting the total mass of the vehicle - it's a win-win because either you can lighten the vehicle if you don't have a minimum weight, but if you are already close to it, you can potentially place ballast where you need it most, or at least place it lower.

I do not know if you are looking at the entire picture but that is how I was choosing my tire size. First you need to get dyno sheet with your power curve. Then you need to find out the optimal gear shift points based on your gears ratios and you will get an idea if you need a bigger of lesser tire diameter as it will affect on overall Drive ratio making gears longer or shorter and affecting maximum speed in each gear. So by changing overal Drive ratio you would try to optimize you power curve combination with maximum RPM and optimal shift points.

As far as rotating mass goes it is quite simple. Please read the following link http://www.w8ji.com/rotating_mass_acceleration.htm

Overall, quite a good guide there, Georg - a couple or three glaring mistakes, but the gist is there.