Wheel offsets explained

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Wheel offset is a measurement taken where hub contacts inside of rim and is relevant to the centreline of the rim and the depth of the dish.

It will be a positive mm or a negative mm or a zero offset.
A zero offset is the centreline of the rim. From this point a positive moves hub contact area outwards and a more shallow dish and negative moves it inwards for a deeper dish.

Standard D22 rims (for eg) have positive 40 offset, this means hub contacts rim 40mm over centre line towards outside of vehicle. If I replace them with a zero offset rim my tyres would stick 40mm further out towards outside of vehicle by way of a 40mm deeper dish. If I went to negative 10mm offset my tyres would stick 50mm further out.

Rim Width.
Standard rim width is 7 inches, if we change to a wider rim, say 8 inch, this extra inch is allowed for by a half inch each side of the offset. I will call a half inch 12mm.

So if I change a positive 40mm 7 inch rim to positive 40mm 8 inch rim, my tyre will stick out 12mm further on my new rim.
If I go 8 inch zero offset my tyre will be 52mm further out and a negative 10mm 8 inch rim would take it to 62mm.

Wider tyres are the same if you go to a 285 from a 265, you get 10mm each side of centreline, so it would not be unreasonable if going from 7 inch positive 40 with a 265, to expect your tyre to move outwards approximately 60mm if going to 8 inch zero offset with a 285, by way of 12mm from thewider rim, 40mm from the offset and 10mm from wider tyre.
 
From the photo you can see I have about 35mm to play with before the bulge of the tyre starts sticking outside of guard. This is the rear wheel and the front has a bit less to play with. Different tyres can bulge more or less depending on compound so your measurements may vary slightly.

Track Width
Track width is measured across the vehicle between the two tyres centreline (i think) and im pretty sure (in WA anyway) your only allowed to increase track width by 50mm so if you wanted to be all legal you may only have 25mm each side to play with but im not 100 percent sure.

Disclaimer
Please check all this info with your tyre/rim supplier before purchasing rims.
If any of this is incorrect feel free to abuse me mercillessly.
 

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I should probably add that best place to start measuring from is hang level like i am and then across to the edge of the rim. After thinking about offset and rim width, then maybe consider tyre width/bulge.
I am measuring bulge as this is relevant to the first part of tyre to start protruding from guard.
 
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Hi some good info there, Just to add, A flange to Flange measurment is from the bolt face of the hub(where the rim sits up against) to the bolt face of the Hub on the other side, usually measured with wheels off and a couple of rulers across the face then the distance measured with a tape measure. Yes this is not entirely accurate as with the wheels off the suspension hangs, but thats how its done.
I know in some States you can get pinged for having tyres outside the Guards, this is the actual tread and does(should) not include the Buldge or sidewall.
Cheers Adam
 
The other one that a lot of people get wrong is that rim width is measured from the inside of the lip not the outsides
 
A pic that will help add clarity.
The bottom of the pic shows a right hand wheel in positive, zero and negative.
 

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