Rim size

In 17x8 ET35, the diameter is 17 in and the width is 8 in. Discount Tire says both measurements run from bead seat to bead seat, not across the outer rim flanges. ET35 identifies a +35 mm offset.

Discount Tire defines wheel offset as the distance between the mounting surface and the wheel’s centerline. For this example, backspacing is 5.88 in by the formula. The wheel offset calculator shows backspacing alongside diameter, width and offset.

Use the tire rim width chart for the exact tire size and model. Maker tables can list different approved ranges for tires with the same size designation.

The measuring rim is not the entire approved range. Tire Rack says it is the rim width used to obtain the tire’s published specifications. These LT tire examples show approved ranges and, where noted, measuring rims:

• 265/70R17: Nitto lists 7.0–9.0 in for the Ridge Grappler, with an 8 in measuring rim. Firestone lists 7.0–8.5 in for the Winterforce LT.
• 275/65R20: Nitto lists 8.0–9.5 in for the Dura Grappler, with an 8 in measuring rim.
• 285/70R17: Nitto lists 7.5–9.5 in for the Terra Grappler G3, with an 8.5 in measuring rim.
• 315/70R17: Firestone lists 8.5–10.0 in for the Destination X/T. Toyo lists 8.0–10.5 in for the Open Country A/T III, with a 9.5 in measuring rim.
• 265/65R18: Toyo lists 8.0–9.5 in for the Open Country A/T III, with an 8 in measuring rim.

49 CFR 571.139 (FMVSS No. 139) requires tire manufacturers to make a listing of usable rims available to the public for each tire they produce. The regulation recognizes rim listings and size designations from publications including the Tire and Rim Association, ETRTO, and Japan Automobile Tire Manufacturers’ Association, Inc.

Discount Tire says to stay within approved rim width and tire size combinations. It warns that mounting outside the approved range can cause irregular treadwear, poor vehicle performance or tire failure.

Tire Rack says section width changes slightly with rim width. Its 205/60R15 example measures 8.23 in across on a 6.0 in measuring rim. The example’s approximate section width becomes 8.03 in on a 5.5 in rim and 8.43 in on a 6.5 in rim.

Discount Tire says a wider rim makes the same tire slightly wider, reduces sidewall flexibility and quickens steering response, but may reduce ride comfort. A narrower rim makes the sidewall narrower and more flexible, softening handling and potentially smoothing the ride. Tire Rack warns that section-width changes can affect fender and suspension clearance; vehicle fit depends on the vehicle, wheels and suspension.