View Full Version : Wheels/tires FAQ. Read this

07-31-2006, 12:32 PM
This thread has been started to cut down on some of the same questions always being asked and to create a good resource for all of us here on Max. Your help is needed and your suggestions of things to add is definitely welcome.

This thread will work very similar to the DIY\FAQ threads in some of the other forums. When you have something to add just IM me with the information.

What is offset/back spacing? (very important) (http://maxbimmer.com/forums/showpost.php?p=848898&postcount=2)
How do I know what tires will fit my car? (http://maxbimmer.com/forums/showpost.php?p=848918&postcount=3)

07-31-2006, 12:51 PM
Offsets and backspacing are very important as far as how the wheels will fit on your car, it is somewhat complicated but if you spend 20mins to understand it, you'll be fine.


The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.

Zero Offset

The hub mounting surface is even with the centerline of the wheel.

The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.

In another word, if your 3pcs ACS wheels was originally 17x7 et 30, by changing the outer barrels to a wider size, you will decrease the offset of the wheels since the centre of the wheel has moved towards the mounting surface.

here's a pic to explain:

Back spacing
Back spacing and offset go together like weed and hookers (may be not), but you cannot calculate your new 3pcs wheels' offset without backspacing, so what is backspacing?

backspacing is simply the measurement of the inner lips to the mounting surface, this measurement will tell you if your new M3 wheels going to rub the coilover shocks or not.

The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace.

What you need to calculate your wheel's offset
So you are wondering why in the world I focus on you need to know your backspacing before you can calculate your new offset, well, you don't want to spend $4k on a set of 18" OZ Futuras with custom barrels then found out they poke out 1" pass the fenders right?

To calculate offset you'll need the following measurements:
-Wheel backspace
-Wheel Width
-Wheel Center line (outboard flange to inboard flange measurement / 2)

-Wheel center line from Wheel backspace to get offset.
-If backspace is less than the wheel centerline the offset is negative
-If backspace is greater than the wheel centerline the offset is positive

To convert from inches to mm multiply by 25.4
To convert from mm to inches divide by 25.4

Wheel offset caculator
This is extremely helpful to know just how your new wheels will fit compare to the current ones. I will explain how important this can be for people going for the Euro/stretch look.

Backspacing offset conversion chart

This is a quick conversion chart

07-31-2006, 01:24 PM
The most frequent questions you hear now days are

Pro'pone: "will 225/25/22 tires fit on my E36?"

here's some low downs on understanding tire sizes and how you know if it'll fit or not.

Here's something interesting, you often see tire sizes starts with a P on most cars, i.e. P205/55/16, the P stands for P-metric, it means the tire is meant for a passenger vehicle, not for heavy duty use etc, there are also other letters which tells you what they are for. But we won't going into that, its unlikely any of you will try to tow 4000lbs with your E30 318.

Section width
P205/55/16, the 205 is section width of the tire, it simplly means it is 205mm wide, to convert it into inches, you simply divide it by 25.4 , which would be around 8", so if you want to fit a 275 tire on your e36, you can find out if the tires is going to fit all simplly take a ruler and measure how much space you have under the fender. HOWEVER, different companies/models have different contact width, we will go into that later on.

Sidewall Aspect Ratio
P205/55/16, the 55 is the sidewall aspect ratio, which indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall.

And you guys know the last 2 digits mean the inner diameter of the tire, or the diameter of the rims.

Internal Construction
This is not necessarily to know if the tire is going to fit or not, but it is useful to know.

A letter (R in this case) that identifies the tire's internal construction follows the two digits used to identify the aspect ratio.

P225/50R16, P225/50ZR16

The R in the P225/50R16 91S size identifies that the tire has a Radial construction in which the tire's body plies "radiate" out from the imaginary center of the wheel. Radial tires are by far the most popular type of tire today representing over 98% of all tires sold.

If the R in the size was replaced with a D (225/50D16), it would identify that the internal tire body plies crisscross on a Diagonal and that the tire has a "bias ply" construction. Tires using this construction are for light truck and spare tire applications.

If the R in the size was replaced with a B (225/50B16), it would identify that the tire body plies not only crisscross the tire on a diagonal as before, but that they are reinforced with belts under the tread area. This type of tire construction is called "Belted." Tires using this construction are practically extinct.

I'm not going to into details about speed rating, I'm sure most of you understand what that means. If not, here's a chart to show you what each letter stands for.

Choose the right size tires
so will 215/45/17 tires fit on my E36 you said? it depends is the width of the wheel, you can't fit 215/45/17 tire on a 10" wheel at most places, simplly it is quite dangerous to seat the beads properly. And if you are not going for the mad euro look, it is simplly pointless.

Each company has spec sheet for each line of their tire models, it will tell you the following:
1) Tread depth, how much tread these tires have when brand new, most are between 10-11/32nd.
2) Tire contact patch width, its the width for the width of the area that the tire comes into contact with the road, it varies from tires to tires.
3) Tire cross section width, its the width from sidewall to sidewall before mounted on a wheel, it also varies from tires to tires.
4) Rim width, this is important, if you are not going with the euro look, follow their spec sheet, DO NOT USE ANY WHEELS THAT ARE NARROWER than minimum width, for example, if the 215/45/17 tire from Manufacture X said rim width is 7" to 8", do not use 6.5" wide wheel, it can be potentially dangerous since the wheels is a LOT narrower than the actualy cross section and may cause the sidewall to "buckle", in another word is you soften the sidewalls too much and the tire cannot perform as the manufacture intent it to be.

so remember, look for the tire spec sheet, you can find them online by searching on Google, every manufacture has them online. Even commercial truck tires.

Another main thing is speedometer accuracy, by changing your tire size/sidewall, you may affect how accurate your speedometer/odometer can read. Since you change the overall diameter it may also affect your acceleration. The rules of thumb are: if you lower your overall diameter, your acceleration will improve, but your top speed will decrease, vice versa if you increase your overall diameter.

Here's a tire size calculator so you know how much overall diameter difference there are between your new and old tires, and usually, if the difference is withing 3% you don't really have to worry about the accuracy, and most BMWs read their speed/odo from the rear wheels

08-01-2006, 04:07 PM
Stuck it.

11-17-2007, 08:50 PM
just want to ask,what do i need to make e36 wheels fit my e34 and were can i get it,i'm asking you cause you seem to know alot about it,thanks

01-11-2008, 06:00 AM
just want to ask,what do i need to make e36 wheels fit my e34 and were can i get it,i'm asking you cause you seem to know alot about it,thanks

some good info here.

Silver Snail
02-12-2009, 08:30 PM
More good info here on remeasuring ET's in case you split your wheels and replaced the lips/barrels.


03-24-2011, 12:13 AM
Ok...so this is "my" question :)......Can i fit 235 tire on my stock 16" BBS....Factory they came with 225/50/16...so can i just fit 235/50/16???

03-24-2011, 12:16 AM
Ok...so this is "my" question :)......Can i fit 235 tire on my stock 16" BBS....Factory they came with 225/50/16...so can i just fit 235/50/16???

That tire size doesn't exit.

03-24-2011, 11:32 AM
That tire size doesn't exit.

Ups my bad :)...i meant 235/60/16?

03-24-2011, 05:21 PM
How wide a tire will fit depends on the width of the rim.

235/60R16 is NOT the proper conversion for 225/50R16

You should be looking at 245/45R16 which would fit IF the rim was 7.5" wide.

But also, tire choices in those sizes is drastically limited, and not recommended at all.

02-18-2013, 07:38 PM
good info thanks