Understanding Wheel & Tire Sizes

The simplest way to buy new tires or wheels that fit your car is to look up the current ones and buy ones the exact same size. If you don't want to do that for whatever reason, you found a deal on craigslist on similar sized ones, for example, or you want to up-size your wheels for looks, it starts to get more complicated. I'll start by explaining the different important dimensions on wheels, and then move to tires.

Wheels:

Wheels have five important measurements:
Image cortesy of sizeitup.com

  1. Diameter. This is the one you're probably most familiar with. It is measured in inches and is usually somewhere between 14"  and 19".
  2. Width. Pretty self explanatory. This is also measured in inches.
  3. Bolt Pattern. This is measured by the number of lugs and the diameter on which they are drilled. It can be measured in either inches or millimeters. This is both important and difficult to measure. The easiest way, if possible, is to find out what car the wheels came from and look up the lug spacing for that model. 
  4. Offset. This is the distance from the centerline of the wheel to the hub-mounting surface and is measured in millimeters. This is pretty important because too small an offset can cause the inside of the wheel to rub against the suspension. Too large an offset is less dangerous, but if too extreme it could cause the tires to protrude from the side of the car.
  5. Centerbore. This is the diameter of the hole in the center of the wheel. This should match the hub size of your car. If the centerbore is too large, it is possible to use hub-ring to center the wheel.
All these dimensions should be specified on any wheels for sale. If they aren't, make sure to ask before wasting time going to look at them. When a wheel's dimensions are called out they will be written as Width x Diameter ET Offset Bolt Pattern. For example, 7x17ET45 5x114.3. These wheels would 7" wide, 17" diamter with a 45mm positive offset. They would have five lugs on a 114.3mm diameter.

Tires:

Tires have many important charactoristics, but only three relate to size:
1. Section width. This is measured in millemeters and is taken from sidewall to sidewall. This will not be the same as the rim width, but there are several online calculators determine what width tires will work on give rim width.
2. Aspect Ratio. This refers to the sidewall height and is measured as a percentage of the section width. Ideally if you upsize a wheel, you will get a smaller aspect ratio so the overall tire diameter remains constant, avoid inacurate speedometer/odometer readings.
3. Diameter. Refers to the inside diameter and must match the wheel diameter.

So, as an example, a 205/65 R16 tire has a track width of 205mm, a height of 65% of 205mm, and fits a 16" rim.

References:

Comments

Post a Comment

Popular posts from this blog

How To Boot Your PC Directly to Steam Big Picture

Image
Valve released their "Big Picture" Steam interface from beta several weeks ago and I am very impressed. Frankly, it puts to shame the dashboards on both the xbox360 and PS3 that have had years of development. I started playing PC games when my dad bought our first Mac with a CD-ROM and I checked out Myst from the library. Despite using the original Xbox for a few years, I've always preferred the versatility of a PC. That  being said I now spend hours a day in front of computer screen at work and plopping down on the couch in front of my 42" HDTV sounds a lot more relaxing than taking on the zombie horde with my mouse and keyboard. Although Gabe Newell & Valve are now in the process of developing their own Steam Box console, I didn't want to wait that long. So with a little Googling and the help of the websites listed at the bottom, I came up with a way to run Steam Big Picture as a shell under its own user account. This post gives instructions on how to
Read more

How to Replace the Oil Pan in a Subaru Impreza WRX

Image
I completed this job on my 2002 Subaru Impreza WRX Wagon (stock) in about 8 hours. That included a lunch break and a trip to Autozone for a 24mm deepwell socket. The vast majority of my time was spent on my back in the driveway trying to reach the bolts around the oil pan, several of which are located up under a frame member. RTV vs Gasket There seems to be some debate about whether RTV or a premade gasket makes a better seal. Most people I talked to advised using RTV, and since that is what Subaru used originally, that's what I did. I was not able to get as clean a bead as I hoped because of the awkward position under the car and I still had no leaks, so I'll add my voice to the opinion that RTV is a good option. Finding the Oil Leak A piece of advice: If you're looking to fix an oil leak, take the time to find out where your oil is leaking for sure and don't just assume its the oil pan. In my case I was pretty sure, because I damaged the oil pan drivin
Read more

Gear Review: Aldi Adventuridge Dome Tent

Image
Aldi is fast becoming my favorite store. After moving to a city devoid of normal grocery stores like Kroger, my wife and I gave  Aldi a try and were instantly hooked by the low prices and convenient layout. Even so, there's a big difference between buying off-brand tomato sauce and trusting your dry sleeping bag to an " Adventuridge " tent. With name brand tents at REI running somewhere north of $300, though, we decided it was worth the risk. After spending three nights (one of steady rain) in the tent, I can say it is more than worth the $60 price tag. The rain flooded the older 3 room tent our friends were using, but our Aldi tent had only small puddles under the windows. It's heavier than anything I would drag backpacking, but its perfect for car camping, and could probably even work for canoeing. The Good: Bathtub floor keeps the water OUT. High ceiling lets adults stand up inside. Room to comfortable fit three adults, a Pack-N-Play, and al
Read more