If you’re shopping for new rubber or checking fitment, the number “305” in a tire size like 305/70R17 refers to the nominal section width in millimeters. That’s the widest point of the tire when mounted and inflated, measured sidewall to sidewall—not the tread contact patch.
What Does A 305 Tire Measurement Actually Mean?
The three-digit number at the start of a tire size always represents the width. This is the standard across brands and is the figure used on tire specification sheets and retailer listings.
Regardless of the other numbers in the size, the width component stays consistent.
Do 305 Tires Vary In Actual Width?
Yes, the true mounted width can differ slightly from the nominal figure. Tire manufacturers design each model with its own casing shape and tread profile, so two different 305 tires mounted on the same rim might measure a few tenths of an inch different. Inflation pressure also plays a role—a lower pressure can make the tire bulge slightly wider at the sidewall.
For most practical purposes, planning around 12.0 inches is accurate. If you’re squeezing a 305 into a tight wheel well or checking clearance against suspension components, you should verify the actual width on the specific tire’s published spec sheet before committing.
Wheel Width Compatibility For Common 305 Sizes
Rim width determines how a 305 tire actually sits. Mounting a 305 on too narrow or too wide a wheel can affect handling, tire wear, and even safety. Here’s the manufacturer-approved range for several common 305 sizes:
| Tire Size | Min / Max Wheel Width |
|---|---|
| 305/45R17 | 8.0 in / 10.5 in |
| 305/65R17 | 8.5 in / 11.0 in |
| 305/70R17 | 8.0 in / 9.5 in |
| 305/40R18 | 10.0 in / 12.0 in |
| 305/60R18 | 8.5 in / 11.0 in |
Using a 305 outside the listed range for your exact size can compromise the tire’s bead seat and alter handling, so always check against the published specs for the tire you’re buying.
305 Width Vs. Overall Tire Diameter
A common mistake is confusing the 305 width with how tall the tire is. The width number alone tells you nothing about diameter—you need the full sizing code like 305/50R20. In that sequence, 305 is the width in millimeters, 50 is the aspect ratio (sidewall height as a percentage of width), and 20 is the rim diameter in inches.
When you’re confirming clearance on a lifted truck or a lowered car, check both dimensions. If you’re ready to see specific 305/30R26 options with fitment details, our tested roundup covers 305 30R26 tires built for performance wheels.
FAQs
- Is a 305 tire the same as a 12-inch tire? A 305 tire is nominally 12.01 inches wide, so it’s effectively a 12-inch-wide tire. The exact mounted width can vary by brand and rim width.
- Will a 305 tire fit on a 10-inch wheel? It depends on the specific size. Some 305 sizes like the 305/40R18 require a minimum 10.0-inch wheel, while others like the 305/70R17 top out at 9.5 inches. Check the tire’s published range before mounting.
- What does the 305 number mean in tire sizes? The 305 is the nominal section width in millimeters, measured at the widest point of the tire’s sidewall when mounted and inflated.
References
- TireSize.com. “Tire Size Chart.” Lists equivalent widths and rim-size matches for a 305 mm section width.