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I just picked up some 20 x 9inch wheels. What would you suggest I use for tire sizes and lowering suspension? I'm a complete noob when it comes to this stuff, so any information would be greatly appreciate.
 

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You need to keep the Circumference the same as the tires that came on the car. If you change the Circumference the speedometer will not be correct. Larger wheels will require a lower profile tire.

Here is a site that you can put in the old tire size and check the new tires for the Circumference and other facts.

Custom rims, wheel tire packages
 

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Beware ... Doing straight math to calculate diameter (or circumference) is not necessarily accurate. For instance ... The same sized (per math) Michelin MP4S will be smaller than the original Contis. The Goodyear run flats are also larger.

If you have a 4 Matic care must be taken off you are changing only one axle. You may need to mentally adjust your speedometer reading if changing tires.

In my case (going Conti to Michelin) my tire shop sent the first set back and got a different size that matched the tires they were replacing.

It turns out equally specified sized tires from the same manufacturer can vary in actual size by model type as well.
 

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I used that when I bought my MP4S tires that had to be returned. Just saying that you need to verify and use caution.
 

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@cinemotive : What TIRE SIZE is on the car now? Need to know what tire size was on the car from the factory. The important thing is to match the original factory tire diameter. We don't know what car you have, or what tires it came with from the factory.

And I agreed, don't use the "size" numbers to calculate the diameter! Find the claimed number in the documentation somewhere. I've been using the data from the manufacturer's websites, making note of the rim size used to get the dimension.

My first set of snow tires finally arrived, so I'll see how they measure out after they are mounted up. I'll also be ordering up a set of lightweight rims for race rubber that are 9.5 wide, so I'll see how those numbers come out as well. I'm actually more concerned about the side clearance values, switching to the wider rim. I'll need to do a bunch of measuring of all the clearances and plug the numbers into one of those handy website chart thingy's.
 

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The factory 20" wheels on my 2018 GLA45 are 235/40-20, so that could be a starting point. But again, start with what size is recommended for your car (since we do not know what that is) and match that diameter as close as possible.
 
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