What is Wheel Backspacing? | Revv.ly Glossary

What is Backspacing?

Here's what frustrates me about wheel fitment discussions: people use backspacing and offset interchangeably, as if they're the same thing. They're not. They're related, but they measure different things, and confusing them leads to wheels that don't fit. Let me clarify this once and for all.
Backspacing is the distance from the wheel's mounting surface to the inside edge of the wheel (the edge closest to the brakes and suspension). It's measured in inches. Offset is the distance from the mounting surface to the wheel's centerline, measured in millimeters. Different measurements, different reference points.

The Relationship

For any given wheel width, backspacing and offset have a direct mathematical relationship:
Backspacing = (Wheel Width / 2) + (Offset / 25.4)
Example: A 9" wide wheel with +35mm offset
Backspacing = (9 / 2) + (35 / 25.4) = 4.5 + 1.38 = 5.88 inches
Alternatively:
Offset = (Backspacing - (Wheel Width / 2)) x 25.4

Why Backspacing Matters

Backspacing tells you how far the inside of the wheel extends toward the brakes and suspension. When you're checking clearance, this is often what matters most:

• Will the wheel clear the brake caliper?
• Will it contact suspension components during compression?
• Is there room for the inner wheel lip?
  Lower backspacing = more room between inner wheel and suspension = wheel sits further outboard
  Higher backspacing = less inner clearance = wheel sits further inboard

The Confusion

Offset has become the standard specification because it relates directly to the wheel's centerline, making comparisons easier across different widths. A +45mm offset puts the mounting surface 45mm from center, regardless of whether the wheel is 7" or 10" wide.
Backspacing, however, was the original American measurement and remains common in the truck/off-road world. Some enthusiasts prefer it because it directly indicates inner clearance without calculation.
The confusion arises when someone gives you backspacing for a wheel and you try to compare it to an offset specification--or vice versa. The numbers look completely different because they're measuring different things.

Practical Application

When researching fitment, note which specification is being used:

• Most performance wheels are specified by offset (mm)
• Many truck and off-road wheels use backspacing (inches)
• Some resources mix them (frustratingly)
  If someone says "I'm running 4.5 inches of backspacing," that tells you about inner clearance. If they say "+30mm offset," that tells you about centerline position. Convert between them using wheel width if you need to compare.

The Takeaway

Both measurements are useful; neither is better. Offset relates more directly to how the wheel sits relative to the fender. Backspacing relates more directly to inner clearance. Know which you're discussing, convert when necessary, and don't conflate them.
The Revvly community uses both terms correctly--join us to discuss fitment without confusion.
Related: Offset, Poke, Tuck
Fitment Resources: Forum Fitment Galleries, Wheel Calculators