Originally Posted by pyro_la
i have a 91 300E with the same tires, 225/45/17 and no rubbing issues, in fact i have alot of clearence. the more i read about this topic the more i think these cars are all unique when it comes to tire and suspension setups. ive heard people getting a 1.6" drop with no problems and then ive herd even people putting a drop of 1.2" with rubbing issues. it seems these cars are all very picky when it comes to these issues. and at the end of the day, its mostly just a trial and error routine.
note: going from a 45 profile to a 40 your only making a change of half a centimeter (5mm) which is not a real big difference. you may want to concider going to a 215/35/17 that may solve your problem, but its still not a garantee.
Completely incorrect. the difference in tire height is not 45 - 40 = 5mm
P-Metric tire sizing works as follows:
Section width (in mm)/ Aspect ratio - rim diameter (in inches)
Sections width: width of the tire from sidewall to sidewall
: height of the sidewall experssed as a percentage of the section width.
Rim diameter: self-explanatory
With the numbers:
255 = section width
45 = aspect ratio
17 = rim diameter
to figure out the overall diameter:
255 + (255 x 0.45) + (17 x 25.4)
= 255 + 114.75 + 431.8
= 801.55mm = 31.56"
as far as the difference in height of a 255/45-17 vs. a 255/40-17
255 x 0.45 = 114.75mm
255 x 0.40 = 102.00mm
The difference in height between the two is 12.75mm or 1/2". Not 5mm
If you were to take the difference between a 225/45-17 and a 255/45-17 you're looking at 43.5mm (758.05 vs. 801.55) or almost 1-3/4"
People typically run into rubbing issues because they don't understand that there is a lot more to tire and rim selection than they think.
Not only are width and diameter important but also offset and the shape of the BACKside of the rim.
Offset (ET) is defined as the distance from the centre of the rim to the mounting face expressed in millimetres. If a rim had an ET of 0mm this would mean that the part of the rim that contacts the hub is exactly 1/2 way in between the two outside edges of the rim.
A rim with ET = 34 means that the mounting face of the rim is 34mm OUTBOARD of the centreline. in other words, the rim sits 34mm further in
than a rim with ET = 0. However, you have to realize that ET is an absolute measurement. which is to say that a rim which is ET 34mm does NOT
sit 34mm further in than a rim which is ET 12mm. In this particular case, the ET 34 rim sits only 22 mm further in.
Another thing to keep in mind when you're trying to figure out how much further out/in a rims sits in comparison to another the rim width.
Here's an example:
OEM rim for a 190E 2.3-16 is 7x15 ET44
OEM rim for a 190E Evolution is 8x16 ET34
How much further in/out will the EvoI rims sit than the OEM 16v rim?
ET44 - ET34 = 10mm further out
BUT you have to take the difference in rim width into account as well.
The EvoI rim is 1" wider overall. Divide by 2 (because 1/2 of the difference goes to each side of the rim's centreline.) and convert to mm = 12.7mm
So 10mm + 12.7mm = 22.7mm further out.
On the flip side (literally) you have to consider that there is a change to how far in the INSIDE edge of the rim sitsbecause we needto make sure there is clearance to the steering and suspension components.
in this case, the difference in ET moves the INSIDE rim edge 10mm OUT, so you get more clearance but the additional width brings that edge 12.7mm back in. SO, the net result is the inside edge sits 2.7mm further in towards the middle of the car.
Furthermore, you also have to consider if the shape of the caliper well (the backside of the rim that would sit in front of the brake caliper allows enough clearance for the caliper. ESPECIALLY if you're going to bigger brakes.
I've already broken down P-metric sizing and how to interpret it. The most common mistake people make when changing rims to a larger size is that they just carry over the numbers from the previous tire size and get tires with the same section width and aspect ratio, but for a larger diameter wheel.
To illustrate the point:
OEM tire size for a 2.3-16 is 205/55-15
Working the math you get a diameter of:
205 + (205 x .55) + (15 x 25.4)
= 205 + 112.75 + 381
if we were to make the common mistake when going to a 16" rim, we'd incorrectly order a tire that is 205/55-16.
Since the other numbers stay the same, logic would say that the overall diameter would be 1" (25.4mm) taller. I won't go through the equation here, but feel free to try it out.
So, obviously a tire that is 1" taller is an issue. This is where plus sizing
comes in. Plus sizing means increasing the rim diameter while maintaining the same overall diameter. Plus-1 means an rim diameter increase of 1", Plus-2 is 2" etc.
the way to figure out proper plus sizing is just simple algebra. using the numbers from above we know that the 2.3-16 OEM tire/rim package has an overall diameter of 698.75mm and we know that we want a 16" rim. assuming that we are happy with the tire section width, we have 3 of the 4 numbers. all we need to figure out is the aspect ratio:
205 + (205 x X) + (16 x 25.4) = 698.75
205 + 205X + 406.4 = 698.75
205X + 611.4 = 698.75
205X = 87.35
X = 0.426
(To those proficient in math: Yes, I know the steps I took were cumbersom, but I did it that way for the benefit of those who may not be as proficient with algebra.)
Since there are no 42.6 series tires, the closst we can hope for is a 40 series tire, but it will be marginally undersized.
"But what if someone wants to change the tire width as well? after all, a 205 tire is really narrow. "
Well, if you know what width you want to go to, be it 225, 245, 255, or even 285, you can figure out the proper aspect ratio to keep the overall diameter the same. Just substitute the numbers you know/want and solve for X.
CAVEAT: this does NOT guarantee that a given tire/rim will fit. This just explains what thought process and math you need to be able to properly compare one rim/tire combo to another. There are other factors to consider such as additional tire width will require more clearance under the bodywork for turning. also watch out for clearance to the shock, tie-rods, etc