Two simple measurements determine the entire layout of your car and a good part of its character: The wheelbase is the distance between the front and rear axles, and the track width is the distance between the wheel centers of an axle. Both help decide whether a car feels stable or nervous, whether it glides comfortably or turns agilely. And you can influence one of these two measurements yourself when buying wheels: You can change the track width via offset and wheel spacers, within the limits of what the documentation allows. This guide explains both terms in detail, their effect on driving behavior, and the simple calculation behind every track change.
Note: Track width (the body dimension) and "toe" in the sense of axle geometry (toe-in/toe-out) are two different things that are constantly confused in everyday life. This article deals with track width; axle geometry belongs to wheel alignment and is only mentioned here for differentiation.
1. The terms clearly defined
The track width measures from tire center to tire center of an axle, perpendicular to the direction of travel. Front and rear axles can have different track widths, which is completely normal by design. The wheelbase measures from the center of the front axle to the center of the rear axle, in the direction of travel. Neither of these values is found in the standard fields of Registration Certificate Part I, but rather in your vehicle's EC Certificate of Conformity (CoC) and in the technical data sheets; the article Vehicle Registration Explained shows which fields the certificate itself contains.
To reiterate for clarification: When the workshop "adjusts the toe," it's about toe-in and camber, i.e., the angles of the wheels relative to the road, not the track width. Incorrect axle angles can be recognized by the tire wear pattern, as described in the article Tire Wear; a changed track width, on the other hand, can be seen from the outside by the flush fit of the wheels.
To put the magnitudes into perspective, as rough guidelines across the market: Car track widths usually range between about 1.4 and 1.7 meters, from small cars to wide SUVs. For wheelbases, small cars typically range from 2.4 to 2.5 meters, compact cars around 2.6, mid-size cars and many SUVs between 2.7 and 3.0 meters; vans and pickups can be even longer. Even a few centimeters difference noticeably changes the character, which explains why two similarly sized cars can feel so different.
2. How wheelbase and track width affect driving behavior
The wheelbase is the vehicle's steadying influence: the longer it is, the more calmly the car reacts to bumps and load changes, the better its straight-line stability and ride comfort, and the more interior space there is. A short wheelbase makes it nimble and agile but sacrifices composure; that's why small cars feel more skittish than sedans. The wheelbase cannot practically be changed afterward; it is a design dimension.
The track width is the stability lever: A wider track reduces body roll, increases tip-over stability, and makes steering more precise because wheel loads are distributed more favorably in corners. Visually, it provides a flush, solid stance in the wheel arch. The downsides: more unsprung movement at the steering wheel on bad roads, in extreme cases more wear on wheel bearings and joints, and, of course, the limits of wheel coverage and clearance. This is precisely where wheel choice and wheel spacers come into play.
That the lever works is shown by the manufacturers themselves: Sports versions of the same model regularly come from the factory with a wider track than the basic versions, visible in the flared wheel arches. The aftermarket essentially does the same thing, just retrospectively and with its own documentation.
3. The math: how ET and spacers change track width
Every track change follows a simple formula. The offset describes how far the mounting surface of the wheel is from the center of the rim: If the offset decreases by 10 millimeters, the wheel moves 10 millimeters further outward, and the track width increases by double that amount, because both wheels on the axle move. An example: original rim ET45, new rim ET35, resulting in 10 millimeters per side and 20 millimeters more track width on the axle.
Wheel spacers (spacer discs) achieve the same without changing rims: A disc between the hub and the wheel acts like an ET reduction by its thickness. Two 15-millimeter discs per axle result in 30 millimeters more track width. Important are the system type (bolt-on spacers centered by the bolt circle or through-hole systems with longer bolts) and proper centering to prevent vibrations. The complete installation and registration process is covered in the wheel spacer guide; the assortment with over a thousand variants can be found in the wheel spacer collection.
View wheel spacers for your vehicle
4. Limits: Wheel coverage, clearance, documentation
Wider isn't arbitrarily wider. Three limits set the framework: First, wheel coverage, because the wheel must still be covered by the bodywork or approved attachments after widening. Second, clearance inwards and outwards, from the strut to the wheel arch edge, in every steering and compression position. Third, the documentation: wheel spacers and wheels with a different offset require their ABE (General Operating Permit) or parts certificate for the specific combination, possibly with an amendment inspection at an officially recognized testing center.
Then there's the issue of load capacity, which is often overlooked: More leverage on the wheel bearing means more stress, and the spacer manufacturers' documentation therefore limits thicknesses and combinations per vehicle. Those who respect the limits get the solid stance without side effects; those who ignore them invite vibrations, rubbing tires, and trouble during approval.
5. Calculate it yourself: the most common misconception about rim width
A persistent misconception: "Wider rim equals wider track." Not true. The track width depends on the wheel center, and that is determined solely by the offset. A rim that is one inch wider with the same offset grows half inwards and half outwards; the wheel center remains, and so does the track width. Only the combination counts: a wider rim plus a lower offset pushes the center outwards and widens the track.
The calculation for your own project, in an example case: Original wheel 7.5 inches wide with ET51, desired wheel 8.5 inches with ET35. The ET difference of 16 millimeters fully contributes to the track (32 millimeters per axle), plus the wheel extends 12.7 millimeters further inwards and outwards. Outwards, this adds up to almost 29 millimeters more wheel position per side, and exactly this number must be checked against the wheel arch edge and clearance. If you also plan to use spacers, simply add their thickness to the ET difference. This way, any combination can be simulated on paper before anything is ordered.
6. Track width in practice: three typical scenarios
Flush fit for road vehicles: The most common goal. Usually, moderate spacers or a rim with a slightly lower offset are enough to bring the wheels visually flush with the wheel arch edge. The effect on driving behavior remains subtle, but the appearance changes significantly, especially in combination with concave designs, whose geometry we explain in the article on concave rims.