Tyre offset refers to the distance from the mounting surface of a wheel to the hub mounting centre of the wheel.
Wheels can have a negative offset and positive offset. if you have wide arches in wheels and tyres, you might need a wider offset so during cornering it doesn't rub against the bodywork due to the wrong offset. On modern vehicles which are developed using computers and engineering software, there will often be components that need to work within very tight parameters. Typically manufacturers want all suspension within +/- 2mm of each other at full travel.
The offset of wheels becomes even more important so that they sit in the right place on all four corners as per the wheel's width.
It is very important that your new wheel has an appropriate positive offset for your vehicle and correct PCD pattern otherwise it can cause all sorts of problems like accelerated tyre wear and poor handling. A good example of this would be if you bought a set of 18" wheels with a +45mm positive offset. These are typically designed for large saloon cars with large mounting surface which have deep arches; however if you put them on your sports car/ vehicle near the edge of the wheel or hatchback with shallower arches with positive wheel offset, you'd find that they sit too far out towards the arch making it impossible to get rid of negative camber in the side of the wheel.The tyre of your vehicle that won't track along the road as well and will wear out much more quickly than it would if it were sitting in the correct place on the centerline of the wheel.
When you fit wider wheels to a vehicle, you need to stick with a specific offset otherwise things like negative camber will become worse, but also your turning circle can increase dramatically. Wheel width almost always requires a change of offset too.
Depending on how much of a difference your negative offset makes, you might need new wheel bearings to better align with the centerline of the wheel. This is especially true if you're moving to wheels that are more than 1 or 2 sizes larger than what you currently have fitted. The bigger the wheel, the greater the effect of even small amounts of offset will be so it's worth thinking about carefully before making your purchase.
It's generally accepted that the optimum offset for an alloy wheel is one that sits in the line of the wheel with the outer edge of its tyre. So if you have a +45mm offset wheel, the tyre would sit 45mm towards the outside of the wheel compared to where it mounts on your hub/suspension. This helps to reduce negative camber of the back edge of tyres, increase contact patch and also keep everything within specification so the suspension is working within its design parameters for the wheel offset. Some car manufacturers specify a low wheel offset for certain models. This is because they don't want tyres rubbing on suspension components when you go over bumps or around corners. It can be difficult to get by with these lower offsets but sometimes making sure you buy wheels that are at least close to the recommended negative offset (if not even a touch higher) can help save you money and time in the long-run and protect the side of the wheel.
A proper wheel offset example would be to look in your handbook where there's complete wheel offset explained and see what it recommends as a minimum/ideal wheel offset for your vehicle along the mounting surface. This way if you stick within 2mm offset measurements, or thereabouts you should avoid causing yourself any problems down the line of the wheel. You can then bump that number up by 5mm in wheel track for a more aggressive look in wheel size which will reduce negative camber and give you plenty of space between new wheels and suspension.
Offset is a measurement that describes how much the centreline of your wheel sits behind where the hub/suspension mounts are. If you have 25mm offset wheels fitted, that means that the hub will sit 25mm further back from where you see the alloy normally mounted on your car. This helps to reduce negative camber and increase contact patch as well as following vehicle specification. Some people think wheel offset is something to do with fitting tyres and how the wheel sits along the overall width but actually, tyre size has nothing to do with offset of the wheels at all. It's worth remembering though that using too low an offset can cause problems in mounting face like excessive wear, poor handling and even rubbing on suspension.
Wheel backspacing is very similar but not the same as wheel offset. Wheel backspacing is measured from the hub mounting face to the plastic rim protector on your alloy near the edge of the wheel in the wheel arch. It's basically how much space there is between the outside of your wheel and this extrusion around it along the wheel.
If you have wheels with a large backspace in front wheel drive cars, they're going to stick out from the car more than those with less space, meaning that they'll have reduced negative camber.
The reason manufacturers recommend sticking within a certain wheel offset and backspacing is because it helps to ensure that your new 20-inch rims rims only just fit into the wheel wells with too much positive offset. This way they're protected, your suspension can move as normal and you'll be able to avoid any rubbing problems along the mounting surface.Using too much positive camber and wrong backspacing measurements can cause bad handling, excessive steering wear and even snap the studs on your alloy if it's severe. This is why you want to keep track of factory settings tweaking them for either looks or performance.
The wheel offset is usually on the outer edge of your wheel and has a 4-5 digit number on it. This is your offset/backspacing measurement. Most aftermarket brands will write '25' or '50' etc on their wheels to denote their size.
It is possible to change wheel offset very slightly by using different wheel spacer washers between hub and wheel but you don't want to go much more than around 2mm from zero offset either way. You can buy space washer and wheel bolts at many accessory outlets and they're pretty cheap to purchase if you decide that you need a bit more negative camber in your life.