Wheel and Tire FAQ

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Wheel and Tire FAQ

Post by Admin on Tue Nov 19, 2013 5:26 pm

As many people do when they get new cars a lot of us are swapping out the stock wheels for aftermarket setups. I have noticed a lot of people don't understand the numbers when it comes to wheel/tire purchases so here are some knowledge bombs. Feel free to add to this.

Wheels

Wheels come with 5 important measurements
Diameter
Width
Offset (Sometimes they give you backspacing and you have to calculate offset or find a table)
PCD
Hube Bore

These 5 numbers will help you decide what will fit on your new car and not rub on the fenders, suspension components, the brake calipers or if they will even mount.

For our cars the PCD should always be 5x4.5(inches) or 5x114.3(mm) if you get something different you will need an adapter/spacer to run them
Also for our cars the hub bore should be 67.1mm or larger. If it is larger you will want to get hub centric rings with an inner diameter of 67.1 and an outer diameter that matches the wheel.

Now for the other three:
Diameter: The measure across the center of the wheel face from one rim edge to the other (this plays a role in tire sizing)
Width: The measurement across the barrel from the inner rim to the outer rim (this plays a role in tire size and offset)
Offset: The measurement from the center line of the wheel to the mounting surface. (the center line is the line down the middle of the barrel, it occurs at half the width, the mounting surface is where the wheel connects with the hub of the car when mounted)

The lower the offset the more pushed away from the vehicle the wheel becomes. Spacers are used to help lower the offset by pushing the wheels away from the vehicle as well.

Now depending on your suspension setup (Stock, Lowered, Slammed, Bagged) and where you want the wheel face to sit (Poked, Tucked, or Flush) some of these measurements will have to change from stock. There are few people on the forums who have created setups that are very flush with the fender, one option is to either copy their width and offsets for your setup or use and offset calculator to see how yours may vary from it. I use Online Wheel & Tyre Fitment calculator. Offset and Tyre Stretch

Wheel weight is also important. Increased wheel weight will reduce gas mileage, and power to the ground and braking ability. Increasing wheel diameter will often cause wheel weight to go up while also moving the weight further from the hub thus making the aforementioned faults even worse. Lighter wheels will increase the power put down, mileage and braking ability but can come at the cost of strength. Forged wheels which are generally the strongest and lightest also come at a higher cost. Finding a good balance for your budget and local roads is the best route.

Tires

Tires comes with 3 measurements as well
Width (usually in mm)
Sidewall Ratio
Wheel Diameter (usually in inches)
Which is shown like this W\SwR\WD

Width: The measure of the tread width of the tire, while the number is the same the actual width will vary slightly between manufacturers
Sidewall Ratio: Take this number put a decimal in front of it and multiply it by the width to get the sidewall height
Wheel Diameter: This number needs to match the diameter of the wheel you want to use the tire on.

When buying tires the wheel width will come into play because a tire that isn't wide enough will cause the sidewalls to stretch, this has become popular in many enthusiast crowds as it allows the owner to lower the vehicle more without rubbing the tires on the fenders. If the tire is too big it will hang out over the wheel too much and risk bubbling and bursting. There are suggested wheel width ranges for each tread width.

Also with the sidewall height being related to the width of the tire it also needs to be considered. The overall diameter of a tire should never be more than 3% different than stock. If it is the ECU will need to be adjusted to calculate for the change. Going 3% larger may also call for upgraded braking systems to account for the added rotational mass. So as your wheel diameter goes up your sidewall ratio should go down, generally by 5 for every inch increase in the wheel diameter. Now if you plan to go with a wider tire you may need to lower the sidewall ratio another 5 to account for it. For example on our cars if you go to a 245 width tire or greater the sidewall ratio would need to be lowered so that its height does not become too tall. I use Tire Size Calculator - tire & wheel plus sizing for tire sizing as it will let you know when the tire is too far off.

Good luck with finding your new wheel setup!

http://www.miata.net/garage/tirecalc.html

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