How To Measure and Install Custom Wheels and Tires: 1973 Mustang Mach 1
Maybe your stock tires finally wore out, or you want to upgrade to new rims and tires or maybe you’re going racing and want to use taller/wider tires. Regardless of the reason that you’re going to a larger tire you’ll have to measure your vehicle's wheel clearance to make sure it will fit properly before you spend any money (because wheels and tires are rarely returnable).
The process of measuring for a larger wheel and tire combo is simple, but you have to be knowledgeable and precise. Don’t forget the old adage: measure twice and order once.
This is what a 20-year-old, dangerous to drive tire looks like. Yes, the tread depth is OK, but because of the age of the tire the rubber has become hard. Note the cracks in the rubber and the large chunks missing from the tread blocks. Yes, it could probably be driven around town, but any aggressive driving will lead to a blow-out.
I planned to put taller and wider tires in the rear and wanted to make sure that there was plenty of clearance. First, I checked the outer fender lip in relation to the existing tire. Stationary, there seemed to be room, but with a taller and wider tire, I anticipate that when the springs compress there may be contact between the two parts that could damage the tire. Rolling the lip of the fender was necessary and I used an old aluminum Louisville Slugger bat against the tire for leverage. This is a body modification that’s very crude, and I don’t recommend doing this if you’re not already planning to paint the vehicle later.
Inner Wheel Well Clearance
Jack up the vehicle with a floor jack and support the frame with a jack stand for safety. Verify that the jack stand is placed in the correct spots—many cars are unibody and there may be weak areas. Pro-tip: Partially elevate the vehicle and loosen the lug nuts before wheels completely leave the ground. Remove the wheel. Once the wheel is removed the measuring can begin. First, get a couple of rulers that are about 12 inches long (yes, the rulers I chose were too long, which is why I’m suggesting that you use something shorter).
Then, align one of the rulers against the wheel mounting surface of the drum/rotor and position the other ruler at a perpendicular (90-degree) angle outside the drum or rotor. Anything that comes in contact with the second ruler is the limit for the size of the new wheel/tire combo. Continue measuring the entire area the wheel will occupy.
Outer Wheel Well Clearance
Reverse the process and measure from the face of the rotor to the inside lip of the fender, and pay close attention to the wheel well and fender lip. I anticipated that there might be an issue and rolled the inner fender lip so I wouldn’t have to worry about damaging the new tire or the sheetmetal.
Measuring for Rim/Wheel Hub Clearance
Next, measure the center cap width.
Then measure for center cap height. Repeat this procedure for the both the front and rear of the car. There will be differences. I'm only going for a bigger wheel in the rear, so I didn’t worry about measuring the front spacing.
New Wheels and Tires
Using Nitto’s website I found that there are handy tire size charts that include mounted tire width, height and optimal rim width. With this information I was able to figure out what tire size would work best with the room I had available. I reviewed the various tire styles and found 275/50/R17 Nitto NT555 G2s for the rear and 245/45R17 for the front. While I plan to drag race the car in the future, I needed a tire that would also do well on the street, which is where it would be most often, which is why I went with the NT555 G2s instead of the NT05R or NT555Rs. Given the room I had in the wheel well and the optimum rim width provided by the Nitto website, I ordered a set of four US Mags 17x8-inch rims with 4.5 inches of backspacing.
Comparing the Old With the New
Here is a comparison of the front wheel dimension measurements. The old tires have about an 8-inch wide tread patch and are 26 inches tall, while the new tires are just a little under 26-inches tall and 9 inches wide. I now have lots more traction and a much better compound.
My old rear tires had a contact patch that was about 9 inches wide and 26.75 inches tall while the new Nitto NT555 G2s are 10 inches wide and almost 28 inches tall. That’s a considerably more rubber on the ground than the previous set of tires.
Due to the fact that I did my homework and took the time to take measurements, I had no issues mounting the tires on the car. Knowing the measurements also gave me options on how I would purchase the wheels and tires. If I hadn’t done the measurements myself, I would only be able to get stock replacements that wouldn’t look or perform as well as what I was able to get. I could also find the lowest prices for the components I chose because I could shop around and have them shipped to me.
Measuring myself also reduced the chances that I would need to return anything. Fortunately, rims and tires aren’t like clothes and manufacturer measurements can be trusted.
Here’s the end result: The new setup better fills the wheel well and will increase my traction both for acceleration and cornering. Plus, the new rims and Nitto tires update and enhance the look of my car. I chose a 17-inch rim because tire sizes are plentiful and inexpensive, and I preferred to keep some semblance of the old hot-rod look with some sidewall showing so it doesn’t look like rims riding on rubber band.
Look for a report on the performance of the tires in the near future.