You may have noticed ratings on tyres that read “6J x 14 H2 ET45”. The “6J x 14” part of that is the size of the wheel rim - in this case it has a depth of 6 inches and a diameter of 14 inches (see the section directly below on wheel sizes for a more in-depth explanation).
The “J” symbolises the shape of the tyre bead profile. The “H2” means that this wheel rim is a double hump design (see hump profiles, below). The “ET45” figure below that though symbolises that these wheels have a positive offset of 45mm. In other words, they have an inset of 45mm. In any case, the info is all stamped on the outside face of the wheel which makes it nice and easy to read and explain for you. On most aftermarket wheels, they don’t want to pollute the lines and style of the outside of the wheel with stamped-on information - it’s more likely to be found inside the rim, or on one of the inner mounting surfaces.
It is easy to understand the different between your old and new wheel and tyre combination in terms of the offset and how it’s going to affect the overall lateral position of the wheel and tyre. Technically you can do this calculation with either the wheel width in inches or the tyre section in mm. I chose to use the tyre section instead of the wheel width because in almost every case, the tyre extends beyond the wheel rim so it’s the widest part of the wheel and tyre combo. So whilst calculating the clearance just with the wheel might show no interference between the suspension or bodywork and the wheel, once a tyre is added, it could interfere as the tyre is wider.
Do you ever Match your tyres to your wheels? Okay. This is the time take a break, relax and then when you think you’re ready to handle the complexities of tyre matching, carry on. Wheel sizes are expressed as WWWxDDD sizes. E.g. 7x14. A 7x14 wheel is has a rim width of 7 inches, and a rim diameter of 14 inches. The width is usually below the width of the tyre for a good match. So a 185mm tyre would usually be matched to a wheel which is 6 inches wide. (185mm is more like 7 inches, but that’s across the entire tyre width, not the bead area where the tyre fits the rim.)
These three measurements are all important to consider when talking about wheels and tyres. They’re all interlinked. Straight from the manufacturer, a tyre is circular and it’s outer diameter (OD) is based on this unladen, perfectly circular condition. Because tyres deform under load (flatter on the bottom than they are on the top when taking the weight of a car), the vertical radius of a tyre under load is not half the diameter.
It’s typically about 44% of it (from the centre of the wheel to the road). In addition, a tyre doesn’t roll like a solid wheel; the steel or fabric belt ‘rolls along the ground’ like a caterpillar tank tread does. Because of this, the rolling circumference of the tyre isn’t quite what you’d expect. The closest approximation for this value can be calculated by taking the outer diameter, subtracting twice the tread depth and multiplying by PI. Roughly speaking, that’s 0.96 x OD x PI.
Now why is this magical value so important? If the rolling circumference changes because you’ve mismatched your new wheels and tyres, then your speedometer will lose accuracy and the fuel consumption might go up. The latter reason is because the manufacturer built the engine/gearbox combo for a specific rolling circumference. Mess with this and the whole thing could start to fall down around you.