What is offset? Can someone explain?
 

I keep reading the offset on wheels but would like to get a better understanding on it. Would someone care to explain that? For example, what is the difference between 30mm and 35mm, and why a ET30 won't work on your car if it requires a ET35? Is the offset of your car listed in your owner's manual? I don't seem to be able to locate it. Or you have to pull the wheels off to get the offset? Lastly, the spacer. How to determine what spacer to use on the offset? Thanks in advance.

Wheel Offset
 

Google search on Wheel Offset and you will have mucho explanatory information.

www.google.com

Steve

You take the WIDTH of the rim and find the centerline dead center of the rim side-to-side, then compare THAT centerline to the flat part of the hub of the wheel (the mounting face that contacts the flange of the wheel hub that the wheel bolts screw in to). The difference between these 2 points is the offset.

Gilly

Where do you look up the factory offset for your car? I have a '95 SL.

Should be in the owners manual in the "technical specifications" section under "wheels and tires" or just "wheels".
Gilly

Gilly explained the offset pretty well. However, in more basic terms, the more the offset, the more the wheel sits inboard. So, if you have a wheel with a 25 offset, like on the SDL, and attempt to use a wheel with a 49 offset, like on some 124's, the wheel will be too far inboard and the tire will hit the tierod.

The 5 mm difference between 30 and 35 mm should probably not be an issue.
If your vehicle requires ET35, and you put a wheel with ET30 on it, the wheel will be 5 mm outboard of the original wheel. This will cause no issues.

If you need spacers, then the width of the spacer should be the difference between the offsets of the original wheels as compared to the new wheels. In the case of the SDL, I'd need 24 mm spacers to get back to the original wheel position if I want to put the 124's wheels on it.

The main question is whether the replacement wheel will hit the tie rod or not.
Of course, if the vehicle is designed for a high offset (say 49) and wheels with a low offset (say 25) are added, the wheel is going to be nearly 1 inch further outboard. This will probably cause contact between the tire and the fender on heavy suspension travel.

TireRack has this diagram: http://www.tirerack.com/wheels/tech/offset.htm

on my car, the offset with standard-sized wheels (6" wide) is 49mm. When I upgraded to a 7"-wide wheel, the offset was 36mm.

The difference of ~13mm (49-36) roughly corresponds to 1/2", which is (7"-6")/2. This meant that in relation to the wheel hub, the position of the inboard edge of the 7" wheel is the same as that of the 6" wheel, and with the outboard edge of the new 7" wheel now protruding 1" more outwards. This offset was adopted for the 7" wheel so that the inboard side of the tire would not rub against the strut.

the rule of thumb is to have an offset as close as possible to the stock setting, of course subject to limitations that would otherwise cause contact of the tire with the inboard or outboard portions of the wheel well (the rear wheels are not as restrictive as the fronts, because they do not steer). As a result of my new offset, my front tires are more susceptible to tramlining on uneven road surfaces, a small tradeoff for the benefits of the upgrade (for example, I now have a track that is 2" wider than stock).

No Offset Spec In Manual
 

I have a 1982 300D. The owner's manual only specifies the following for wheels: 5 1/2 J X 14 H 2. Do you know what that translates as to offset? Thanks!

I can't recall the exact number for the W123, but, it's very close to 25. So, any replacement wheels that you are contemplating need to be no more than 30 or they might hit the tierod.