Posted on by

Wheel Rim and Tire Specifications Explained

What do those numbers on the side of a tire mean? Are your rims 15″, 16″, 17″, 18″ or bigger? What’s an offset and why does it matter? If you are questioning if those new rims you picked out will fit your ride then this guide is just for you. Let Pacific Motors take you on a refresher of the basics needed to choose the perfect replacement or upgrade wheels and tires for your special vehicle.

Tires Tire Size is expressed in a string of numbers and letters like: “255/50R19” 255 = width in millimeters 50 = A percentage ratio of the width divided by height of the sidewall. R = Radial, the type of tire construction (most common type on modern vehicles) 19 = Diameter measurement in inches, must match the diameter of the rim to be installed on

Tread depth is the measurement of remaining tread surface on a tire. A gauge measures the distance from the surface of the tire to the deepest part of the tread. Measurements of 4/32nd Inches or 3mm and greater are considered safe. Measurements below 2/32nd or 1.5mm must be replaced immediately.

Tread depth can be estimated without a guage as well. Insert the penny upright into a tread groove. If you can see all of Lincoln’s head above the rubber then tread is less than 2/3nd or 1.5mm and no longer safe to drive on.

Wheels Wheel Rims are the metal body that support tires and bolt on to the vehicle hub. Wheels are commonly made in two ways. One-piece rims are a solid forged metal construction. Advantages of the one-piece design are strength and performance. Disadvantages are fixed dimensions and difficulty repairing when bent. Two-piece wheels consist of a wheel face and inner barrel held together with bolts. Three-piece wheels work similarly, but with the addition of another barrel piece sandwiched in the middle. Advantages are cosmetic appearance and customizable offset and fitment using different barrel dimensions. Prices are often more than double that of forged rims. Wheels are commonly made from steel or an alloy of aluminum or magnesium. Steel is about 3 times heavier but also less likely to crack or bend. Alloys are lighter and dissipate heat better but are more fragile.

Wheel Measurements Specifications are usually stamped on the inside of OEM wheels. Alloy formula, part number, dimensions, load limit and offset are commonly included. Diameter is the measurement in inches between bead not including the rim flange. Width is the measurement from the inside of the bead on each side of the rim from front to back. It is difficult to measure with a tire installed but can be estimated by measuring the tire width and rounding down to the nearest 1.5-inch measure if dimensions are not stamped on the inside of the rim.

Bolt Pattern is the number and spacing of hub bolts. Bolts range from 3 to 12 or more and are arranged in a ring. Most passenger vehicles use 5 or more bolts. 5 bolts arranged in a ring with a 120mm diameter is expressed as 5x120, the standard bolt pattern for BMW vehicles. Measuring the diameter of the bolt ring by hand will only provide an estimated size. A special tool is used to get true measurements. The bolt pattern for most vehicles can be found by searching online. Lug Nuts come in three common varieties. The differences come from the seat surface which contacts the wheel. The designed used on the wheel will determine what type of nut must be used. Conical (tapered, acorn) lug nuts have a smooth cone on the inner mounting surface. Ball (radius, rounded) type have a bulging seat. Flat (washer, flange, mag) type have a straight 90-degree seat like any common bolt nut. Using the wrong type of lug nuts will prevent the wheel from torqueing to the hub properly and can be dangerous.

Offset is a measurement from the centerline (mid-point of width) to the mating surface where the rear of the rim touches the hub when bolted on. Maintaining an offset close to the original is important to prevent steering problems or wear on bearings. Offset measurements are noted as a number followed by the letters ‘ET’ such as - 37ET. To measure offset, place the wheel face down on a surface. Lay a straight edge across the rim on the bead flange. Measure from the bolt mating surface to the straight edge. This measurement is called backspace. Now take the rim width and divide by 2. The resulting number is the centerline. Subtract the centerline from backspace. A negative number indicates a negative offset. High positive offset improves stability while high negative offsets allow more design space for spoke effects. Offset will also affect the appearance of vehicle stance.

Centerbore is the opening machined in the center at the back of a wheel. This hole aligns the wheel on the hub and helps position it for torqueing the lug nuts. Measurement is taken as the opening diameter and stated in millimeters. OEM sizes should be maintained when possible. Aftermarket adapters are available to convert from a larger to smaller centerbore.


Are we forgetting anything?

If you have any unanswered questions about how to pick the right rims and tires please comment below and we will do our best to fill you in!