Is a 31" tire really 31"

[SteveVB]

Something that has been discussed here before is the nominal size of a tire. Mostly relating to why one person can run a certain size while another can't. While looking for new tires I ran across the following dimensions. Thought it might be of interest.

ALL FOR ADVERTISED 31/10.50-15 TIRES

Super Swamper SSR ..dia 31.5", BF Goodrich AT ..30.75", BF Goodrich MT ..30.86", General AP .. 30.5", Mickey Thomson MTX ..30.2".

Not really new info, but I was suprised by the range of sizes. Interesting that the Super Swamper SSR at 31.5" is only about 1/4" smaller than the BFG 32/11.50-15 at 31.8", and I would bet that there are some 32" tires that are smaller than some of the 31" . These are advertised Dia on a 8 or 8.5" rim. Once the tire is on a loaded vehicle the Loaded radius will be different also. The adv. dia is valid for a relative comparision.

Just food for thought

Steve
91 Navajo

 

[GJarrett]

Just as interesting is the comparison test Peterson's OffRoad magazine did earlier this year on several 33" tires. According to them, all of those tires measured a true 31", not 33". So a 33 is really a 31.

 

[NoBoundaries]

Hey Steve,
For the most par t a tire is not the exact size it states. The most accurate tires are the metric tires. A lot of people do not understand how to interpret the metric tires into inches, but I explained to another Explorer owner how it worked. Unfortunately the tread was closed so I couldn't post it to help other how to interpret the metric tires into inches, so I guess I'll take the opportunity now. This is part of an e-mail I sent explaining the metric system: These tires for the most part are no different from metric tires, the only difference would be what type of tire you get. Let me explain to you how metric works (It also should be noted that metric tires are measured more accurately than tires measured in inches. A 32 inch tire is actually about 31.5 inches and you can go to a tire company's web site and they will usually say this under "Overall Diameter", BFG for some reason does not have this). I'll describe the metric system by explaining a size such as the 235/75R15, usually the stock size tire on most Explorers other than the 255/70's. The 235 is the width of the tire in millimeters. To change this number into inches for easier interpretation you just need to divide this number by 25.4. In that case your tires are about 9.25 inches wide. Now this is were it may get complicated. The 75 stands for 75% (percent). The 75% is the percentage of the width (9.25 inches) that the side wall is high. That is from the top of your rim to the top of your tire. So you take 9.25 x .75 (75%) and you get the number 6.938976378 or relatively equal to 6.94. Now you have the length of the side wall in inches (6.94 inches). Now you add 6.94+6.94 (You can think of this as adding the top sidewall and the bottom sidewall of the tire, above and below the rim) which equals 13.88. Now the easiest part. The R15 stands for the radius of the tire, which is equal to the radius of your wheel, 15 inches. You take that 15 inches and add it to the 13.88 inches and you have 28.88 inches. This is the size of your tire in inches which really has no application to the purchase of your new tire. When you want to buy a new tire, for the most part the shop just needs the make and model of your vehicle and they will find what size you need, or you can just tell them 235/75R15 and they will find the tire for you. I hope this helps you.

Tim Y.
1996 Ford Explorer
FordAdventurer@aol.com

 

[SteveVB]

Tim

My point about the sizes is that you need to check with the manufacturer of the tire you are planning to use to find the size. I know all tires are not as advertised, I was just suprised at the range.

You did a great job of the calclations for a metric tire but once again I could have titled the post "Is a 265/75r16 really a 265/75R16. If you check some of the manufacturers specs, you will find ranges for the 265/75-16 range from 31.2" to 31.8" tall, the calculations work out to about 31.65.
If you are going to change tire sizes you need to check the specific manufacturer and style of tire that you are going to use. I believe this may be one of the reasons that so many report that such and such size tire fits on their vehicle while another using the same size tires have some problems.
The size of the tire is specific to the brand and model of tire.
Also new vehicle owners with ABS sensors at all 4 wheels need to stick with the same size tires at all positions. So running different brands front to rear or replacing just two tires seems to be a thing of the past. I read in a tire dealers mag that even four tires from the same manuf. and the same style can be off enough to confusue the computer. Although I dont remember whow far off is off.

Steve VB
91 Navajo