What offset and backspacing mean
Offset is the distance in millimetres from the wheel's centre line to its mounting face, usually stamped as ET. A positive offset means the mounting face is outboard of the centre, which is normal on most modern vehicles. A lower offset pushes the whole wheel further out of the fender.
Backspacing is the distance from the inner edge of the rim to the mounting face. It equals half the rim width plus the offset, so it grows with both a wider rim and a higher offset. Backspacing is what usually decides whether a wheel touches the suspension.
Worked example
A 7.5 inch wide wheel with ET45 becomes an 8 inch wide wheel with ET40. Half the width grows from 95.3 to 101.6 mm, while the offset drops by 5 mm. The inner edge ends up 11.4 mm closer to the suspension and the outer edge sits 1.4 mm further out, so the extra width goes mostly inboard. Backspacing rises from 140.3 to 141.6 mm.
These figures are illustrative. Measure your own clearances before buying, because a calculation cannot see a brake caliper or a strut.
Why offset matters beyond looks
Changing offset changes the scrub radius, which affects steering feel, tramlining and behaviour under braking. It also changes the load path through the wheel bearings and hub, and a large change can accelerate wear.
A wider track can put the tire outside the bodywork. Check that the tire stays within the fender and that the change is acceptable to your insurer and your state's inspection rules.
Measuring clearance
Turn the steering to full lock both ways and check the gap between the tire and the inner fender, the strut and any brake components. Compress the suspension or load the vehicle to check the fender lip at full bump.
The inner edge figure from this calculator tells you which way the wheel moves. It does not tell you how much room you already have, so measure that gap on the vehicle first.
Limits of this calculator
It models rim width and offset only. It ignores the tire's section width, which can be wider than the rim and is often what actually rubs, and it ignores flange thickness, spoke profile and brake caliper shape.
For the overall diameter and speedometer effect of a tire change, use the wheel size calculator or the tire size calculator.
For overall wheel diameter and circumference, use the US wheel size calculator.
For UK spelling and tyre terminology, use the UK wheel offset calculator.
Inputs for the US wheel offset calculator
Fitted wheel width
Rim width in inches, usually stamped on the wheel as 7.5J rather than measured across the outside.
Fitted offset
The ET figure in millimetres. Positive means the mounting face is outboard of the centre line.
New wheel width
Rim width of the wheel you are considering, in inches.
New offset
The offset of the new wheel in millimetres, taken from its stamping or specification.
CALCULATOR QUESTIONS
Wheel offset calculator US questions
Does a lower offset push the wheel out?
Yes. Reducing offset moves the mounting face inboard relative to the rim, so the whole wheel sits further out of the fender.
Is a wider wheel always further out?
No. Half the extra width goes to each side. With the same offset the outer edge moves out by half the increase and the inner edge moves in by the same amount.
Can I change offset without changing width?
Yes, and the effect is simpler: the whole wheel moves by the difference in offset, in or out.
Will this tell me if the wheel fits?
No. It tells you which way and how far the edges move. You still need to measure clearance to the suspension, brakes and fenders.