I’ve been seeing a lot of wheel/tire threads and since I’ve had issues with people and stores recommending wheels and tires that don’t fit properly, I have been trying to learn as much as possible about wheel and tire fitment.

This is by no means a definitive tire and wheel guide; it’s just some things I thought might help while choosing wheels and tires for your cars.

Starting with the original equipment size, I use this Tire Size Calculator to compare tire sizes. This calculator will let you know if you are going over 3% of the original tire size. I also go to Discount Tires website, where you can enter your vehicle and see the original equipment size and plus sizes they recommend. I’m pretty sure there are other sites you can find out the original tire size, this is just my preference.

Here’s something I found at Yokohama’s web site that helped me to understand the relation of tire size to wheel width:

Choosing a wheel near the middle of the range will give a balance between ride quality and handling. A wider wheel will improve handling at the expense of ride quality, while a narrower wheel will improve ride quality at the expense of handling. Consider these compromises when selecting wheels.

You can get the recommended wheel width range from the tire calculator I mentioned above.

When it comes to wheels, it’s pretty much a matter of what size tire you choose and what will fit your car. The offset of the wheel is very important; it determines how much backspace the wheel will have. This in turn determines how much of the wheel will extend from the hub to outside your wheel well opening.

What I usually do is when I look at a wheel offset; I convert it from millimeters to inches using a conversion calculator then add that to half of the wheel width (measured from outside lip to outside lip). This will basically give you the backspace of the wheel. There are some wheel manufacturers that use backspace instead of offset in their wheel descriptions. Once I get the backspace, I subtract it from the total wheel width to see how much of the wheel will extend from the hub to the wheel well opening.

I went as far as making a plumb bob (a string with a big nut hanging from it, pressed up against the fender at the center of the wheel well opening) to measure the distance from the mounting surface of the hub to the outside of the wheel well opening. On my 06 GT, it is 3.75 in.

If you have open spoke wheels, this is easy enough to do. In some cases you may have to pull the wheel to be able to measure from the hub mounting surface to the wheel well opening.

To give you an idea how it all comes together…

I started with my 06 GT’s original tire size 235/55/17 on 17x8 in wheels. I went to the Discount Tire website and entered my vehicle. I decided I wanted to go +1 and chose 255/45/18. I opened up the tire calculator, entered my original tire size, then the size I chose from Discount Tire.

By comparison, this tire is very close to the same diameter as the 17", will have a little less sidewall, wider tread and will keep speedometer close to stock. The recommended wheel width is 8-9.5" for the 255/45/18, I chose to go with a 9” wheel, good balance of handling and ride quality. Not to mention the 9” wheel will give the wheel well a fuller look.

Now it was time for wheel shopping. Some of you may have seen my post about my experience with an ebay store that sold me wheels with a 24mm offset. If we do the measuring and calculating, the wheel extends a little over 4 in. from the hub mounting surface. This would put the wheel approx. ¼ of an inch outside my 06’s wheel well opening (that’s just the wheel), this is unacceptable and isn’t the right fitment for this car.

I then found some wheels I liked that have an offset of 38mm. By my calculation the wheel would extend 3.5 in. from the hub to the wheel well opening. This is more to my liking.



Good Luck with your tire and wheel shopping.

 

You deserve..like..to be King Wheel Fitment or something for this.

 

Thanks

If I find anything else that may be helpful, I'll post it here.

Have a great day.