My years of involvement in the automotive industry has seen many trends come and go but, the 1 thing that doesn't change is the enthusiast. You might be a lowrider from Chico, an SCCA autocrosser from Topeka or a 4 wheeler from Moab it doesn't matter, stock is just not good enough for you, or me for that matter.
In this post I am addressing tire load capacity because I often get asked about tire sizes which are just not appropriate for certain applications.
In general, I have been asked about shorter tires for a car more often than taller tires. When I ask "why do you want a tire which is that much shorter?" the normal thought is that they are looking for a performance upgrade or ride height adjustment. I rarely find people who have even considered load capacity.
If you are looking to improve your gear ratio for more acceleration out of the a corner when at a local autocross shorter tires are not the best option in most cases. Bagged and need to "put it in the weeds" again shorter tire may not be the best answer. Your hopping hydraulics do not need a shorter tire to operate properly either. These three completely different performance desires have enthusiasts often looking for shorter tires which have a lower load capacity.
Tire load capacity is directly related to overall diameter. It is a fact that shorter tires have less weight carrying capacity.
Let's consider the idea of running a capacity that is too low from a performance point of view. The tire is put into a position where it can support the car at a static rest point but, when driven aggressively the sidewalls are so over worked they can not properly control the contact patch of the tire which allows for excessive sidewall roll-over, irregular wear, tread block and contact patch flex. These all rob the tires performance ability and grip. The added flexing of the tire will increase the heat it generates which can make a tire feel really slippery. The better gear ratio is achieved but if you can't utilize the power it doesn't matter.
A good way to understand this is: let's say I just put a 100lb. back pack on you .... you can stand there for quite a while (that's your car sitting in a parking lot). Now, go run up and down some stairs with that back pack .... that's normal driving and you can't do that very well, nearly as fast or for very long before your legs give out. Next, after you've recovered enough to walk, (let the car rest once you arrive at the venue for your autoX), take a 500 meter sprint through deep sand (that's your first run at the autoX) and you will most likely not make it to the finish without falling at least once if you can finish at all.
*** Load Capacity is Important! ***
My name is Luke Pavlick and I am a Car Guy