I currently have 205/45 ES100's on s-heavies. I've been going through some calculations between these and 15 or 16" wheels. My sidewall should be 3.6" (205*.45/25.4) but I measured it at less than 2.5". What gives? I checked the wheels on one of our other cars and those are also a little shorter than the calculated height. Is it safe to assume that, for the same tire and same overall diameter, the sidewall will change by 1/2 inch for every inch of wheel diameter change? (ie. a tire with a 2" sidewall on 18's will have a 2.5" on 17's, 3" on 16's, and 3.5" on 15's)

 

No, it is not a linear equation equaling 1/2 inch per inch of wheel diameter because there is also relationship between the tread width (1st number) and sidewall height (2nd number). If you do an internet search using 'tire size calculator' there are quite a few hits that will give you a solution when you plug in the tire variables.

 

Yes and no.

Each tire is a bit different than calculated values. Each manufacturer varies with how they construct the tire and how it fits on the actual rims.

Here's the problem- with each rim size there are limited choices for sidewall heights. The larger the rim the more lower sidewall choices are possible and some are pretty short indeed. Then you can choose tires that are more narrow like 205 or more wide like 225mm.

You can get the most choices and greatest range of possible sidewall heights with smaller rims especially 15" and 16".

15" rims use
175/65-15
185/65-15
195/60-15
205/55-15
205/50-15
225/50-15
225/45-15
Depends on the width of the rims from stock to 15x7 or 15x8.

17" rims use
205/45-17
205/40-17
215/40-17
215/45-17
225/45-17 a bit larger than stock.

Adjust accordingly.

 

Thanks. I am trying to get a better picture of how my sidewall would change if I go from 205/45-17 to 225/45-16 or 225/50-15. I've been on the Miata tire calculator and just wanted to compare what they say the sidewall *should* be with what it actually is. I didn't expect it to be over 1" off. Do manufacturers measure sidewall height from the bead to the tread or the rim to the tread?

 

Go to Tire Rack, look up a specific tire, and the click the "Specs" link. It'll give the actual diameter of that particular tire. Simply subtract out the diameter of the wheel and then divide the result by two to get the height of your sidewall.

Example:
http://www.tirerack.com/tires/Spec.j...e1=yes&place=9

BFG KDW2 in 215/45/17 has a sidewall of 24.3" - 17" wheel diameter = 7.3". 7.3"/2 = 3.65" sidewall height.

In 205/50/16, it has a sidewall of 24.1" - 16" wheel diameter = 8.1". 8.1"/2 = 4.05" sidewall height.

225/45/16 (one of your examples) has a sidewall of 23.7" - 16" =7.7". 7.7 / 2 = 3.85".

Don't forget that changing the overall diameter of the tires will also affect the accuracy of your speedometer.

 

Well my current ES100's have a diameter of 24.3" and my sidewall is not 3.6" ...

 

Are you measuring just to the outer edge of the wheel for your sidewall height? I think the one inch difference you are seeing is that maybe you should measure to the inner bead seating surface of the wheel and the tire to be getting the true wheel and tire diameters. Does that make sense?

 

The sidewall height extends into the wheel rim a bit.
If you measure the outer parts of the rim, you'll probably find that
it measures 18.25 rather than 17 inches (my 16 inch rim measures 17.25).
Based on the picture. your sidewall is probably closer to 3.4 inches including
the bead portion that's in the rim.

 

Ah, thanks for the info. That's what I wanted to know. My s-lites are 18.5" edge to edge so they're covering up almost .5" of sidewall all around.

 

actually, they're covering up 0.75" of the sidewall all around.

 

the best way to calculate is to use the formula that manufacturer's provide on the sidewall itself:
205 (tread width in mm) / 45 (percentage of tread width that creates sidewall height) /17 (wheel size)
translated:
205mm/92.25mm/17"

each manufacturer varies in sidewall height as well as tread width... in other words a toyo 205 might be a few mm wider than a yokohama.