How to use the fitment studio
- Enter your current tire size, wheel width and offset. They’re usually on the tire sidewall and stamped on the back of the wheel (for example “7J x 16 ET45”). If you’ve saved the car in My Garage, use “Fill from My Garage”.
- Enter the new setup you’re considering, including any spacer.
- Press Calculate. The numbers, a 2D cross-section and the 3D view all update together.
- Spin the 3D view, switch between Both, New and Current, and use the preset angles to look from outside, front or top.
- Use Print or save as PDF for a one-page fitment sheet, or copy a link to share the comparison.
This combines the tire size calculator and the wheel offset calculator in one place, so a change of tire width and wheel offset at the same time is shown correctly.
Worked example: 16-inch to 17-inch
Current: 205/55 R16 on a 7-inch ET45 wheel. New: 225/45 R17 on an 8-inch ET40 wheel. The wheel’s outer edge moves +17.7 mm, and because the tire is also 20 mm wider, the outer face of the tire moves +15.0 mm while the inner face moves −5.0 mm. The overall diameter grows 2.4 mm (+0.38%).
Taking it to the shop
The printed fitment sheet lists both setups and every change in one place, which makes it easy to ask the right questions:
- Is this tire size approved for this wheel width, and does it meet the vehicle’s load index and speed rating?
- Do the wheels match the vehicle’s bolt pattern, centre bore and seat type?
- Will the outer edge moving +15 mm, or whatever your figure is, clear the fender at full compression?
- Will the spokes clear the brake calipers?
- Will the tire pressure sensors transfer, and does the speedometer need recalibrating?
How it works
wheel centreline (mm from hub face) = −(offset − spacer)
wheel edges = centreline ± wheel width × 25.4 ÷ 2
tire edges = centreline ± section width ÷ 2
tire diameter = rim × 25.4 + 2 × width × aspect ÷ 100
change = new − current (+ = outward or taller) The 3D model uses the same numbers: the tire profile is drawn from its nominal width, sidewall height and rim diameter, and the wheel from its width and offset. Positive offset follows Tire Rack’s definition. For the background, read offset vs backspacing and plus sizing.
Frequently asked questions
Is this my actual car and wheel?
No. The 3D model is built from the numbers you enter: tire size, wheel width, offset and spacer. Spoke design, brakes and body are generic or left out, because WheelSignal doesn’t use a vehicle database. It shows how the new setup sits relative to your current one, which is the part the numbers can answer.
What do poke and tuck mean?
Poke means the wheel or tire sits further out than before, toward or past the fender lip. Tuck means it sits further in. The outer tire edge figure tells you how far the outside of the tire moves; the inner figure shows how much closer it gets to the suspension.
How accurate are the tire edges?
They use the nominal section width from the size marking, which is defined on the tire’s measuring rim. On a wider or narrower wheel the real width changes by a few millimetres, and raised lettering or rim protectors add more. Wheel edges are exact for the width and offset entered.
Does it tell me whether the new setup fits?
No. It shows how far each edge moves and how the diameter changes. Whether that clears the suspension, brakes and bodywork through full steering lock and suspension travel can only be confirmed on the vehicle. Print the fitment sheet and take it to your tire shop.
Why won’t the 3D view load?
It needs WebGL, which most modern browsers support. If it’s switched off or unavailable, the 2D cross-section and all the numbers still work.
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