Wheels and tyres Wheel offset (ET)
Wheel Offset or ET
The T25 Offset or ET is 39 for vans that left the factory with Steel wheels, and 30 for vans that left with Alloys
Wheel offset (ET) is the distance from the centre of the wheel to the face of the bolting plate (drum or hub flange face).
Effectively, the offset is also the distance from the centreline of the wheel to the hub face.
Therefore: If you put a wheel on the same hub with a bigger offset, the wheel will sit further inside the wheel arch
So for example... your new rims have an ET of 45...so you will have to fit spacers of 10mm to bring it down to 35 which is within the 39 to 30 range
Alternatively if it has a smaller offset it will sit further out from the wheel arch.
Smaller offsets are very difficult to to fit and are not recommended
Covkid: I disagree on being difficult to fit. I run on Merc steels with ET25. Went straight on and no issues or adverse effects at all. Common mod in Germany.
The offset is described by the term ET, which is from the German word 'Einpresstiefe' translated as 'insertion depth'.
Most wheels have a positive offset which means the mounting surface is further outboard than the wheels centre. If you reduce the amount of positive ET on the wheel the the vehicles track is widened and vice versa if the ET number is reduced. Straying too far from the original offset can be detremental as fouling to the suspension and bodywork can occur as well as excess strain on wheel bolts or studs
Vehicle wheels are usually described with a designation of something like '14-5J' and this is decoded as follows.
The 14 is the diameter of the wheel in inches from edge to edge, measured inside the flange with sizes ranging from 10 inch on the original (Classic) mini's to 23 inch on some 4x4 vehicles.
The second number is the width of the rim, again measured in inches between the flanges. The original mini used 4.5 wheels and sizes range all the way up to 12 as used on some supercars.
The letter, in this case J refers to the shape of the rim. Rim contours are standardized by the Tire and Rim Association, so that tires will fit.
17" wheels are about the limit for T25's and a 7.5J width is also the limit..Rims over these recommendations result in serious suspension issues and rapid tyre wear