If everything was easy, we would not need specialists. Before that it would be necessary to clarify in which direction the series wheel should grow. It can go wide or increase its diameter. Stop, here we have to distinguish between rim and tire. Both can grow, but also each one individually.

Yes, although it is rare, it is even possible for certain off-road vehicle types that only the tire grows or grows more than the rim (pictured above). When the vehicle is rather intended for normal road traffic, however, usually the rim grows and the tire then compensates.

Let's take a particularly simple, unspectacular example, a Peugeot 108. It is available with wheels of dimension 165/65 R 14 or 165/60 R 15. So both times the same width, but different rim diameter, the former with 14 · 25.4 mm = 356 mm, the latter with 15 · 25.4 mm = 381 mm. In the first case, the tire has a height of 165 mm · 0.65 = 107 mm, the other 165 mm · 0.60 = 99 mm.

If you compare the difference in the rims of 381-356 mm = 25 mm with that of 107-99 mm = 8 mm, which must be doubled, however, because it occurs above and below, then one sees a certain relativization of the larger rim by the tire. Instead of 25 mm difference between diameters of wheels remain 25 - 16 mm = 9 mm.

What are the consequences now? One important thing is, the sidewalls of the tires have become shorter. The tire bounces slightly worse, but transfers the desired direction of travel directly to the road. This also affects the steering, which feels appropriate. But the feedback from the road to the steering is a bit more violent. The steering could be a little more nervous.

Now this is relatively harmless in our example. It should also only clarify the direction in which the chassis of a car developes when you combine larger rims with tires of lower cross-section. Incidentally, the dynamic wheel radius could be changed by the stiffer sidewalls so that the determined difference of 4.5 mm (as a radius) turns out to be smaller in driving tests.

This is probably necessary because of the dynamic wheel radius and the transmission between engine and speed depends. And that is not constant anyway, but even differs with increasing speed. That alone might still be bearable, but as long as scanning over ground is not yet introduced nationwide, the speedometer will depend on this radius.

If the differences are caused by optionally mountable wheels in such a way that the speedometer indicates either less or more than 7 percent more, then its ratio would have to be readjusted. So engineers have to rack their brains over possible combinations because of the 5 mm modular dimensions used in the tire industry for tires, one inch for the rim diameter and half an inch for the rim width.

The tire industry also does not have it easy, considering how many different sizes it must have available compared to earlier . Incidentally, the tire width also influences the rolling circumference. We just stick to our simple example and you may then transfer that to your next desired wheel swap.

Thus, if the tire with 165 mm width and the same rim diameter of 14 inches gets 185 mm width, then the tire height increases at the same height-width ratio of 165 mm · 0.65 = 107 mm to 185 mm · 0.65 = 120 mm. The difference resulting for the dynamic wheel radius is almost threefold with 13 mm, by the way here without the additional effect of a shorter and therefore more rigid sidewalls.

The height-to-width ratio would therefore have to be reduced to 0.55, resulting in a tolerable 185 mm · 0.55 = 102 mm. And what is the difference in driving behavior, apart from the already explained stiffer sidewall? It is always easy to say that wide tires offer a greater risk of aquaplaning when wet. At first glance, that is far too fast.

The removal of a possibly forming wedge between the road and tire depends primarily on the negative profile. It would be theoretically possible to give the 185er exactly as much as the 165er has. In practice, one can observe that (true) wide tires actually have wider ruts compared to their narrower colleagues (picture below).

And then comes the direction of the grooves. Against too much water is transversely better than longitudinal. So you can also reduce the risk of aquaplaning. Now please do not ask whether a wider tire has any advantages at all if it puts the same amount of rubber on the road as a smaller one. Maybe it's the broader support that makes a difference here.

In addition, other factors such as the rubber compound are certainly important. A tire solves something like a bundle of about ten problems, the winter tires at least one more. Something must have been done on his improved grip, after all, the braking distances have also shortened by the increasing broadening of the tires tremendously.

Incidentally, consumption and CO₂ emissions do not suffer as much because of a possibly increasing rolling resistance, so the treads have probably become stiffer. The comfort reduction was partly absorbed by the suspension/damping, but has a slightly negative effect on the wheel change on the otherwise unchanged suspension.

However, the disadvantage of greater air resistance has remained. This causes an increasing deterioration of the c_W value the broader the tires selected. You only need to look at corresponding technical data. But what counts all this against a look and the unbridled feeling of rolling over the roads with a completely new vehicle. Especially buyers of used cars see it as a possibility for individualization.