Thinking about a wider, lower-offset or plus-size wheel? Enter your current and new wheel and tire below and the calculator draws both to scale — showing the change in poke, backspacing, overall diameter and speedometer error, so you know whether it will clear the strut and the fender before you buy.
Wheel Offset & Backspacing Calculator
Enter your current and new wheel & tire — the diagram draws both to scale with the fender, suspension and brake for context, so you can see the change in poke, backspacing, overall diameter, speedometer error and clearance before you buy. Tire as width / aspect / rim (e.g. 225/45/17), rim width in inches, offset (ET) in mm.
Advanced: current clearances (for a rub check)
Approximate gaps on your current setup: above the tire (fender), outboard to the fender lip (wheel-well), and inboard to the strut/spring. The tool then tells you how much is left after the change.
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How to read it: a positive inner change moves the tire toward the strut; a positive outer change is more poke toward the fender; a taller overall diameter eats into the fender gap. Keep the diameter within about ±3% of stock, and always test a new fitment at full steering lock and full suspension compression. Clearance figures are estimates from the gaps you enter.
Only changing tire size (same wheel)? Use our tire size comparison & speedometer-error calculator instead.
Offset, backspacing and poke — what they mean
These three numbers all describe where the wheel sits relative to its mounting face (where it bolts to the hub):
- Offset (ET) is the distance in millimeters from the wheel’s centerline to the mounting face. A higher (more positive) offset tucks the wheel further in; a lower offset pushes it out.
- Backspacing is the distance from the mounting face to the inner lip of the wheel. More backspacing = the wheel sits closer to the suspension.
- Poke is how far the outer edge sticks out past the mounting face — too much and the tire rubs the fender or sits proud of the arch.
They are linked: for a given width, lowering the offset reduces backspacing and increases poke by the same amount. Making a wheel wider adds to both sides unless you change the offset to compensate.
Will the new wheels fit?
There’s no single “safe” offset — it depends on your car’s suspension, bodywork and tire size. Use the calculator to see the change versus your current setup, then sanity-check it:
- Inner clearance: more backspacing brings the tire toward the strut, spring and control arm. Even a few millimeters can cause rubbing at full suspension compression.
- Outer clearance (poke): more poke brings the tire toward the fender lip — check at full steering lock and over bumps.
- Rule of thumb: staying within roughly 5 mm of your current inner and outer edges is usually safe; bigger changes may need spacers, a different tire size, or fender work.
Frequently asked questions
Is a higher or lower offset number more “poke”? Lower (or more negative) offset gives more poke — the wheel sits further out. Higher offset tucks it in.
What’s the difference between offset and backspacing? Offset is measured from the wheel’s centerline (in mm); backspacing is measured from the mounting face to the inner lip (usually in inches). This tool shows both and converts between them.
Can I just use a spacer instead of a new offset? A spacer effectively reduces offset (adds poke and inner clearance). The calculator’s “outer” and “inner” changes apply the same way — enter the net offset after the spacer.
Will the new setup affect my speedometer? Only if the overall tire diameter changes. The calculator shows the overall-diameter change and the resulting speedometer error above — keep it within about 3% of stock.