Upgrading 17 in Wheels to 18 in
 

Hello,

I'm very new to cars so bare with me, and I tried to find my question before posting, so I apologize if I didn't dig hard enough.

I currently just have OEM rims/tires for my 2006 Mazda 3 i sedan. I would like bigger wheels though. So my two questions are:

1. As long as I compensate for the bigger rims with thinner tires, do I need to do another other mods to get them to fit?

2. Does anyone have recommendations for where to order them?

Thanks!

Hi Sierra,

Upgrading wheels and tires can be very simple, or it can be rather complex.

Given your current scenario, I'll try to answer your questions as best, and as simply as possible.

1. Yes, if you compensate for the larger wheel with a 'thinner' tire the actual diameter and circumference of the new larger wheel may stay the same. This is actually called the profile of the tire, and is the second number in the tire size. So say you are going from a 205/50/17 and the new 18" wheels have a 215/40/18 tire, the actual diameter and circumference will end up being very close to the original wheel and tire. There's many calculators available online for this that will show you the difference in diameter for the new vs. old tire size and will also provide the difference in speedometer readings that you will now have with new sized tires. Keep in mind however that the profile of a tire is a ratio based on the width (therefore a 205/50 tire may actually be lower in profile than a 355/45 tire, even though the 355 tire has a lower profile number).

In terms of other mods to get them to fit, or issues with fitment, you'd need to be sure you have these things ironed out:

Lug Pattern

Hub Bore (especially if you're going to try to use other OE wheels, for example a Subaru has a smaller bore than Mazda uses, therefore without intensive modification to the wheels, they won't fit).

Width and Offset (Offset is the measurement from the center line of the wheel to the mounting surface of the hub/lugs). Zero is in line with the center line, positive would make the wheel sit further in, negative out. I believe the Mazda 3 uses an offset of ET52.5 (offset is often expressed as ET). Therefore if you wanted to match that, look for an offset of around 52.5. If you wanted the wheels to stick out further, look for something lower. This only applies to wheels with the same width however. So your wheels currently are 17x6.5 with ET52.5. If you replaced the wheels with a 18x7 ET52.5, the wheels would stick out .25 inches further than the original wheels on the inside and outside. This is where you need to be careful. When fitting different sized wheels on a car that is not lowered, your primary concerns would be rubbing on the inside. The biggest chance of this happening is because the wheel is too wide with a high positive offset, leaving them 'sunk in' to the car. The wider a wheel, the lower offset you generally need. Even more complex is the fact that wheel width and wheel offset are in two different units (offset is mm, width is inches). Conversion would be 0.04" = 1mm. SO.... if you go 4 inches wider with a wheel, you'd need the offset to be 50mm lower offset in order to sit the inside of the wheel exactly where the original was.

Now, I realize this is a lot to try and understand right away.

The easiest way to determine if the wheel/tire is going to fit are:
1.) Try it on. This is hard though, especially when buying new.
2.) Don't stray far from your current size. Like you mentioned, counteract the larger wheel by running a lower profile tire. In terms of width, don't go more than half an inch, or an inch wider, and remember, the wider you go above this, the lower offset you should look for.
3.) Go with something you KNOW that will fit. I'll tell you right now that Mazdaspeed6 and Mazdaspeed3 wheels will fit your car without a single issue. This is about as easy as it comes.
3.1) Look around to learn what will fit. Browse all the Mazda forums, there's hundreds and thousands of posts in fitment and wheel spec threads where people share their wheel and tire specs and what modifications had to/didn't have to be done to make them fit.

2.) I have never bought a new set of wheels in my life, so I have no recommendations in terms of where to order. However I will recommend buying used. People buy wheels all the time and then don't like them, their mom/dad/boyfriend/girlfriend gets mad at them for buying them, and you can pick them up for way less than you'd pay. Just beware of cracks or bends, or fake wheels.

Feel free to comment or message with any questions, I'd be happy to help!

So, for example here is my 3. I went from 205/50/17 on a 17x6.5 to 215/45/18 on a 18x7. Since the offsets were so similar, the difference in width had no impact.

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