Derspeed

I have been noticing many members asking what size tires they should choose for their new larger wheels. The easiest way to determine this is to go to tirerack.com and just choose the wheel size you wish to install.

When I started modifying European cars in the early 1990s, however, I had no access to online assistance. I had to do it myself. I developed an easy equation. Obviously I am not first person to figure this out, but I like simple things. Therefore, I made it really simple for myself.

So, if you want to know the overall diameter of your current setup and would like to know if your choices for an upgrade are correct, you can figure it out by yourself if you choose. Here is a very easy method.

Start with your tire size:

225/45-17 indicates that the tire is 225mm wide, and the sidewall height is 45% of the width. This is the aspect ratio. The number 17 indicates that the tire is to be installed on a wheel that is 17" in diameter.

Ready for the easy part?

Convert 225mm to inches by multiplying it by .04
225mm x .04 = 9"

Multiply 9" by 45% or .45

9" x .45 = 4.05"

Now you know the height of the sidewall. The sidewall, however, goes all the way around the wheel. For the overall diameter to be determined, 4.05" must be doubled, giving you the height of the sidewalls at the top and bottom of the tire.

4.05" x 2 = 8.1"

Now all you do is add 8.1" to the wheel diameter, which is 17"

8.1" + 17" = 25.1"

25.1" is the overall diameter of your wheel and tire combo. Now you try it with some of your tire selections. Usually you can safely get away with an OE to aftermarket difference of plus or minus ½” or .50" OD

Let’s recap:

Your tire size is 225/45 - 17

225 x .04 = 9

9 x .45 = 4.05

4.05 x 2 = 8.1

8.1 + 17 = 25.1

Now plug in your numbers.

For those of you who have asked the question, I hope this helps. To those of you who find it too elementary, I apologize.

Krytech

Great timely advise Derspeed!

And for those that failed basic math, here is a website I've used that lets you compare your OE setup to the rims/tires you're considering. I like it because it shows a lot of other details such as what the difference in your speedometer will be, sidewall height, revs per mile, acceptable rim widths, and more. I think you can compare up to four different setups.

C300Sport

Great info guys!

Derspeed

I love that site.

jstaneff

Here's a slight engineering question: What is the effective diameter of the wheel once we take into account the weight of the car. The car presses down the tire, meaning that the distance to the hub for the part that makes contact is shorter than for anywhere else along the wheel's circumference. So, the effective diameter is maybe 2 inches smaller than the wheel diameter.

That has an effect on how the odometer/speedometer reads. The instruments are designed for a specific wheel and tire combination. Increasing or decreasing sidewall height will change the effective diameter, throwing off the gauges.

Also, a very thin sidewall will appear to cause the car to stand taller, and many in this forum would then demand that the car be lowered

We can measure the circumference and consider that so many rotations add up to distance traveled, but I'm thinking that's too simplistic, and that sidewall height (thin/thick) plays a role in this!

amdeutsch

Actually the engineers who designed the car tire/wheel combo took all of that into account. Therefore the original premise of how to calculate +1, +2 holds true with 1 minor caviat: 1" = 2.54 cm (25.4 mm) instead of 2.5 cm (25 mm).

Derspeed

The reason I posted this thread is to help people with a simple equation that shows folks how plus sizing works. There are many variables that impact speedo readings, gearing and so on. And that all changes when the tire tread wears down. Also, a stiffer sidewall will probably not compress as much as a softer one. That's why you generally hear that is okay to be off by about plus or minus .50" OD.

Anyway, if you go to a retailer for a larger wheel and tire package, you can do the math to double check what the retailer tells you. It seems that some folks on this forum have been sold tires that aren't really appropriate.

RLE

...25.1" is the overall diameter of your wheel and tire combo...

Looks more like 24.97 to me.

Derspeed

Yes, yes. How about 24.97247 inches.

You converted 225mm to 8.8583 inches and then multiplied by 45%, doubled it, and added 17"

I was simplifying the process by rounding 0.0394 inches to .04 inches as you would see on a simple conversion chart.

So basically, for purposes here, I think we can agree that 24.97 inches and 25.1 inches are very close?

Jeez.

RobertG

Dont forget the gravitational factor of the moon, full moon will affect the side wall compression as well as a new moon. Best bet take measurment between moon fazes.

RLE

That's more like it.

But, you stopped too soon. What you need to end up with is the percentage difference between stock and some other size. Which means you should continue to revs-per-mile and determine the percentage difference. The experts say that about 3% is the limit before you start seeing significant differences in gearing and recorded vs true mileage.

Keeping in mind that tires that are undersized, thus turning faster than stock, will record more miles thus eating into your warranty miles.

Carsy

RLE, You just must have been/are a school teacher . There is no other explanation.

RLE

Am not, wasn't.

Having done some tire size swapping in my past, mostly on my Porsche, I don't use the same method of calculation that was demonstrated previously.

Derspeed

I wonder how many more final miles on the odometer it would take to deter someone from changing their wheel and tire combo.

I didn't stop too soon. I stopped where it was appropriate to stop based on converting mm to in rounding up to .04. It is a way to quickly check to see if the tires you are considering are an acceptable size. When I shop for plus-size tires, and an expert tells me what I should install, I use this equation to see if the tire size is appropriate. It always is. There have been cases on previous vehicles of mine where a couple of tires with slight size differences would have been acceptable, but not perfect. There was no in-between size. You can't tell a retailer to sell you a tire in some fraction of a size larger or smaller than what is actually exists. You have to buy what is available and as close to the original overall diameter as possible. If a tire size doesn't exist, you have to take what is actually manufactured. If the next size up for your particular wheel diameter and width is all that is available and it's within an acceptable limit, then that is what you will use.

I am not sure why you keep adding these comments regarding issues that don't really apply to what my thread is about.

If you used a different method on your Porsche, start a new thread and share the Porshce method with us. I am always willing to learn more.

I was just trying to offer up a little help for those wanting to upgrade their wheel and tire package. The method, while not your Porsche method, works with real world tire sizes. It worked on my past Audi, Volvo, Jag, Viper, some VWs, Saab and Chevelle.

Good day.

08w204

Quick and easy.... I used to sell tires:

For every 1" increase in rim size, increase the first number by 10 and decrease the second number by 10...

It doesn't work so well when you're dealing with low-profile tires, but in the realm of regularly-sized tires (i.e. everything but sport tires) this formula works very well.

Here's an example of a truck tire: 235/75/15 translates to 245/65/16 and 255/55/17, all pretty closely matched.