How to use it
- Enter the current wheel: diameter and width in inches, offset in millimeters. They are usually cast on the back of a spoke or inside the barrel, for example 17x8 ET35 or 17x8 +35.
- Enter the new wheel the same way. If you only know the backspacing, convert it to offset with the formula in the questions below.
- The drawing cuts through both wheels at the hub, so you can see the change on each side.
- Read the two numbers that matter: how much closer the wheel gets to the fender on the outside and to the suspension and brakes on the inside.
- Below the result you also see the approved rim widths for the new tire size from the comparison page.
Offset and backspacing explained
| Term | What it means |
|---|---|
| Offset (ET) | Distance in millimeters from the wheel's centerline to the face that bolts to the hub. ET, from the German Einpresstiefe, means the same thing. |
| Positive offset | The mounting face sits toward the outside of the wheel, tucking the wheel in. Most cars and crossovers use it. |
| Zero offset | The mounting face sits exactly on the centerline. |
| Negative offset | The mounting face sits toward the inside, pushing the wheel out. Common on lifted trucks and Jeeps for a wider stance. |
| Backspacing | Distance in inches from the mounting face to the inner edge of the wheel. It is about half the overall width plus the offset; overall width is usually about 1 in more than the nominal width because of the rim flanges. |
How width and offset move the wheel
Each extra inch of width adds about 0.5 in (12.7 mm) on each side, and each millimeter less offset moves the whole wheel 1 mm outward. The two effects add up, so a wider wheel with lower offset can move a long way toward the fender. Frontspacing is the outer-side twin of backspacing: the distance from the mounting face to the outer lip.
| 16x7 ET45 | 17x8 ET35 | Change | |
|---|---|---|---|
| Frontspacing (outer lip) | 2.23 in (56.6 mm) | 3.12 in (79.3 mm) | 0.89 in (22.7 mm) farther out |
| Backspacing (inner lip) | 5.77 in (146.6 mm) | 5.88 in (149.3 mm) | 0.11 in (2.7 mm) farther in |
| 17x8 +34 | 17x9 −12 | Change | |
|---|---|---|---|
| Frontspacing (outer lip) | 3.16 in (80.3 mm) | 5.47 in (139.0 mm) | 2.31 in (58.7 mm) farther out |
| Backspacing (inner lip) | 5.84 in (148.3 mm) | 4.53 in (115.0 mm) | 1.31 in (33.3 mm) more room inside |
The second swap is the classic way to make room for wider tires on a lifted truck: the tire gains space next to the suspension, but it moves more than 2 in toward the fender, so trimming or fender flares are often part of the job.
Wheel spacers
A spacer lowers the effective offset by its own thickness: a 20 mm spacer on an ET35 wheel behaves like ET15. Moving the wheel out increases the load on wheel bearings and can change how the steering feels. Use hub-centric spacers made for your vehicle, torque them to spec and check the torque again after a short drive.
Track width, scrub radius and stretched tires
- Track width: a spacer or a lower offset moves the whole wheel outward, and both sides of the axle move, so the track grows by twice that amount. Going from 17x8 +34 to 17x9 −12 moves the wheel's centerline 46 mm (1.81 in) out on each side and widens the track by 92 mm (3.62 in); a 20 mm spacer alone adds 40 mm.
- Scrub radius: the distance on the road between the point where the steering axis meets the ground and the center of the tire's footprint. Moving the wheel out by a given amount increases it by about the same amount. A larger scrub radius makes the steering heavier when parking, passes more bumps and kickback to the steering wheel and makes the car pull more when one side brakes on a slippery patch. The calculator shows the approximate change; the exact value depends on the suspension geometry.
- Stretched and pinched tires: every tire size has an approved range of rim widths. On a rim wider than the range the tire is stretched, with the sidewall pulled tight and the rim lip exposed; on a narrower rim it is pinched, with a bulging sidewall that squirms in corners. For example, a 225/45R17 is usually approved for rims about 7 to 8.5 in wide. The calculator warns when the new wheel falls outside the range for the new tire.
What this calculator does not check
- Bolt pattern and center bore: the new wheel must match the hub, for example 5x114.3 with the right center bore or a hub-centric ring.
- Brake caliper clearance: the spoke shape matters, not only the offset, and big brake kits may need a specific wheel.
- The wheel's load rating, which must cover your vehicle's axle load.
- Steering lock and suspension travel: check with the wheels turned fully and the suspension compressed.
Keep the tire inside the fender. Besides rubbing, tires that stick out can break local rules on fender coverage or mud flaps, so check your state's rules before going wide.
Questions
What is a good wheel offset?
One close to your original wheel, adjusted for width. To keep the outer edge in the same place on a wheel 1 in wider, raise the offset by about 12.7 mm; all the extra width then goes inward.
Is ET the same as offset?
Yes. ET35 and +35 mm describe the same offset. Negative values are written ET-12 or −12.
How do I convert offset to backspacing?
Backspacing in inches is about (wheel width + 1) ÷ 2 + offset ÷ 25.4. A 17x9 wheel with −12 mm offset has about 4.53 in of backspacing.
Does a lower offset make the wheel stick out more?
Yes. At the same width, every millimeter less offset moves the wheel 1 mm outward and 1 mm away from the suspension.