Start with the wheel currently on the vehicle
Poke is how far a wheel’s outer edge sits outward from the hub mounting face. To estimate a donor wheel’s change in poke, compare it with the wheel currently fitted–not with a generic factory specification.
Record both wheels’ width and offset. Wheel width is usually stated in inches between the tire bead seats; offset, marked ET, is the distance in millimeters from the wheel’s centerline to its mounting face. A higher positive offset places the wheel farther inward, all else equal. A lower offset moves it outward.
For example, compare a reference wheel marked 18×8 ET40 with a donor marked 18×9 ET25. The donor is one inch wider and has 15 mm less offset. Those two changes must be considered together.
If the reference wheel has a spacer, include it in the comparison. A spacer moves the wheel outward, so its effective offset is the wheel’s stated offset minus the spacer thickness. A 10 mm spacer on an ET40 wheel, for example, gives an effective offset of ET30 for this geometric comparison.
Calculate the change in outer poke
For nominal wheel width in inches and offset in millimeters, estimate the change in outer-edge position with:
Change in outer poke (mm) = 12.7 × (donor width − reference width) − (donor offset − reference offset)
A positive result means the donor wheel’s outer edge sits farther outward. A negative result means it sits farther inward.
For the 18×8 ET40 to 18×9 ET25 example:
- Width increases by 1 inch, adding about 12.7 mm on the outer side.
- Offset decreases by 15 mm, moving the wheel outward another 15 mm.
- Estimated change in outer poke: 12.7 − (−15) = 27.7 mm outward.
That is roughly 1.1 inches farther outward at the wheel edge. It is an estimate, not a guarantee that the tire or wheel will clear the fender.
The calculation uses nominal, bead-seat width. The wheel’s total flange-to-flange width is greater, and the exact difference varies. So this formula is useful for comparing marked wheel sizes, but it does not locate every lip, spoke, or tire sidewall precisely.
Check the change in inner clearance
The donor wheel’s inner edge can move toward or away from suspension and brake components. Estimate the change in inward reach with:
Change in inward reach (mm) = 12.7 × (donor width − reference width) + (donor offset − reference offset)
A positive result means the donor wheel extends farther inward and may reduce inner clearance. A negative result means it extends less far inward and may create more room.
In the example, the result is 12.7 + (−15) = −2.3 mm. The donor wheel’s inner edge is estimated to sit about 2.3 mm farther out, leaving slightly more room on the inside. This does not prove that the wheel clears the strut, control arm, brake caliper, or other components; their shapes and positions matter.
Backspacing can provide another comparison. It measures from the wheel’s mounting face to its inner rim edge. If reliable backspacing measurements are available for both wheels, comparing them can help estimate the change in inner reach. Use the same measurement method for each wheel, and do not mix backspacing measured to the rim lip with calculations based on nominal bead-seat width.
Treat tire and vehicle clearance as separate checks
Wheel poke is not the same as tire poke. Tire width, sidewall shape, the tire’s approved rim-width range, and the width of the wheel it is mounted on all affect how far the tire extends. Two tires with the same labeled size can also differ in actual dimensions. Check the tire manufacturer’s specifications when available, and assess the mounted tire rather than relying on wheel calculations alone.
Static clearance is only a starting point. Check for interference with:
- Brakes: Caliper clearance depends on the wheel’s inner barrel and spoke profile, not just width and offset. A wheel can have adequate inner-edge clearance and still contact a caliper.
- Steering: Turn the steering through its full range and look for contact with the liner, suspension, or other parts.
- Suspension movement: The tire and wheel move relative to the body as the suspension compresses. Clearance at normal ride height does not confirm clearance under load or through full travel.
- Fenders: A wheel or tire that sits close to the fender may contact it during compression, cornering, or with passengers and cargo.
Also compare wheel diameter, bolt pattern, center bore, lug-seat type, and load rating before installation. These do not change the poke calculation, but they affect whether a wheel can be safely and correctly installed. A different diameter can also change brake clearance and tire diameter.
Decide whether the geometry is acceptable
Use the calculation to screen a donor wheel, then verify fitment on the vehicle. If the estimate shows substantial outward movement, pay particular attention to tire-to-fender clearance. If it shows increased inward reach, check the suspension, brake, and steering-side clearances.
A spacer can move a wheel outward, but it cannot correct every fitment problem: it reduces inner clearance while increasing outer poke, and it changes effective offset. Do not use one to solve an interference issue without confirming all clearances and that the mounting arrangement is suitable. If the donor wheel’s geometry leaves inadequate room on either side, a different width or offset may be the better choice.
The calculation answers one specific question: how the wheel edges are likely to shift relative to the hub mounting face. Final acceptance depends on the actual wheel and tire, the vehicle’s components, and clearance through steering and suspension movement.
