So there's a lot of wheel and tire choices...
Diameters 13", 14", 15", 16", 16.5", 17", 18", 19".... so on and so forth...
Widths from bicycle tires to 395 and even larger...
Sidewall profiles 30, 35, 40, 45.... so on and so forth.. What does that number even mean?
Let's first discuss a general "goal" for replacing the wheels and tires on your ride.
Are you setting your car up for a motor sports application?
If so, what type of motor sports?
If not, what sort of driving will you be doing?
Do you have a ride quality expectation?
All of these are questions you need to ask yourself when drooling over that killer set of three piece forged wheels. No one wants to spend a grand a wheel and the car ride awful afterwards. At least no one with any sense....
You need to determine the primary purpose of the vehicle first. This will guide you in your selection.
For this explanation, we're going to assume you're building a street car to cruise and enjoy. We're also going to assume you know the wheel well limitations in regards to the size that'll safely fit on your vehicle.
The first figure that you need to know is the manufacturer's original tire and wheel overall height. "Why?" you ask. Your vehicle's suspension geometry, wheel arches, speedometer calibration and various other systems were designed with that wheel and tire height in mind.
Simply put, let's say your car's OE equipment wheels and tires stood 26 inches tall. You buy an aftermarket wheel and tire combination that stands 28 inches tall. You also have installed lowering springs to lower the car 1.5 inches.
Your tire now extends 1 inch taller into the wheel well, 1 inch taller in ride height leaving only half an inch of over all ride height lowering and your new wheels and tires contact the inner fender well when you had an inch or so of clearance before.
Is that an exact description of the situation down to fractions of an inch? No. But it is not out of the realm of possibilities because of the size differences.
One last assumption before we move on. It's assumed that you are as OCD as us in regards to the perfectionist mindset that comes with being a fabricator. The stuff we're going to discuss beyond this point will help you find a wheel and tire combination that'll fit with as little headache as possible.
Back to your OE wheel and tire combo being 26 inches tall.
Let's define the three parts of tire measurement using a very popular high performance tire, the Toyo Proxes R888 in 275/40ZR/17.
First, 275 stands for the measurement of tread width in millimeters. This measurement will correlate specifically to your wheel selection and how much space you have for width in your wheel well. A 275mm width tire will fit wheels from 9"-11", is measured at 9.5' wheel width and 11" section width. The actual tread width is 10.4".
The next section is for sidewall. A "40 series" tire is approximately 40% of the tread width tall on the sidewalls. In regards to sidewall, the smaller the number, the shorter the sidewall and the stiffer the tire will ride. This increases "bumpiness" and road noise. A thinner sidewall transmits more from the road.
The last number is simply the rim diameter the tire is designed for. In this case, a 17 inch wheel.
The reason this tire is the example, is because it stands at 25.6" inches tall when mounted and properly inflated, per the specs from Toyo.
So let's put this in to a real world situation.
Your 2004 Mustang Cobra came with 245/45R/17 tires on a 17" wheel. This tire and wheel combination stands 25.7" tall with small variances between tire manufacturers. Let's call it 26" for ease of discussion.
The Toyo above, height wise, doesn't vary widely from the OEM tire overall diameter meaning that your speedometer is still correct, your suspension still sits in the OEM height and the geometry is minimally altered.
For comparison's sake, lets look at a 1993 Mustang GT. The factory wheel and tire combo used a 225/50R/16. A tire and wheel combination that stood 24.6 inches in overall height.
Now, your Toyo stands an inch taller, adding to the overall circumference which would alter your speedometer calibration, even if only minutely. It also stands the tire taller in the wheel well and may come into serious contact with suspension components and the body.
That isn't a big deal for a fabricator. We make things work. But it's good to know these things before you even open your wallet right?
We hope you found a little bit of something that helps in here.
Go do work, son!