How Do I Figure Out a Trailer Wheel's Offset?

Published 10/22/2026

Question:

We have a 2024 grand design solitude 376rd. Im looking for an additional spare tire and wheel. Not sure which offset we need on the wheel. The tires are 215 75r17.5 coopers. The spare is the same size but its a Westlake. Any guidance would be appreciticed. Also, what lug nuts would we need.

asked by: Mark E

Expert Reply:

Hi Mark,

Sadly, I'm not able to tell how much offset the wheel has based on the tire size. A 17.5 tire usually has an offset, it can be positive, negative, or zero depending on if you have dually wheels or a single wheel on each side of the axle. If you look at the wheel there should be the letters 'ET' stamped somewhere followed by a number(s). The number is the amount of offset and there will be a dash before it if its negative. If you can't find that you can also take a measurement with a straight edge and a tape measure.

To determine your wheel's offset, lay the wheel flat with the front facing down and place a straightedge across the back edge of the rim. Measure from the straightedge down to the wheel's mounting surface to find the backspacing. Next, measure the wheel's total width from edge to edge and divide by two to find the centerline. If the backspacing is greater than the centerline, the wheel has a positive offset and sits farther inward. If they're equal, it has a zero offset. If the backspacing is less than the centerline, it has a negative offset and sits farther outward.

Also, are your current wheels steel or aluminum?

expert reply by: Claire H