Rim Width and Tire Size
Each tire size has a range of approved rim widths, published by the tire maker and in the tire and rim standards (ETRTO in Europe, TRA in the US). There is no reliable rule of thumb, so always check the maker’s data for the exact tire. Use the calculator below for the tire’s dimensions on its measuring rim.
Tire dimensions on its measuring rim
The calculator needs JavaScript. The formulas and tables on this page work without it: sidewall = width × aspect ratio ÷ 100, and diameter = rim × 25.4 + 2 × sidewall (in mm).
Overall diameter
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Why there is no chart here
Many sites publish a rim width chart for each tire width. The approved range comes from the tire standards and the tire maker, and can differ between tire models of the same size. Wikipedia: Tire code notes that the TRA Yearbook or the manufacturer’s guide should be checked for the specific tire, and that there is no rule of thumb. So we do not publish a general chart.
Where to find the approved rim widths
- Tire maker data sheets: most makers list the measuring rim and the approved rim width range for each size on their website or in a data book.
- Tire and rim standards: ETRTO (Europe) and TRA (USA) publish the standard tables in their yearbooks.
- A tire shop: can look up the range for the exact tire and your wheel.
How rim width changes a tire
The section width in a tire size is measured on its measuring rim. On a wider or narrower rim within the approved range, the mounted width changes a little, and the sidewall shape changes from more upright to more rounded. Always check your vehicle owner’s manual, the tire placard in the door jamb, or a tire shop before changing size.
Wheel size markings
A wheel marked 8Jx18 is 8 inches wide between the bead seats with a J flange profile and an 18-inch diameter. The wheel also has an offset (ET in mm) that moves it in or out in the wheel arch, which matters for fit as much as width.
Frequently asked questions
What rim width do I need for my tire size?
Check the tire maker’s data sheet for that exact tire: it lists the measuring rim and the approved rim width range. A tire shop can also look it up. We do not give a general rule because the standards and makers say there is none.
Can I put a wider tire on my rim?
Only if the wider tire’s approved rim width range includes your rim width, and it fits your vehicle. Check the tire maker’s data and ask a tire shop.
What does 8Jx18 mean on a wheel?
8 is the rim width in inches between the bead seats, J is the flange profile and 18 is the diameter in inches. The offset, marked ET, is given separately in millimetres.
Does rim width change the tire diameter?
Only slightly. It mainly changes the mounted section width and the sidewall shape. The size code dimensions apply on the tire’s measuring rim, and the maker’s data sheet gives the measured values.
What is a measuring rim?
The rim width on which the tire’s published dimensions are measured, defined in the tire standards for each size. Mounted on a different approved width, the measured width differs a little.
Where are ETRTO and TRA tables?
In the yearbooks published by the European Tyre and Rim Technical Organisation and the Tire and Rim Association. Tire makers’ data sheets usually list the relevant values for their own tires.
Last updated: October 15, 2026