FYI: what all those letters mean
Four Wheeler Mag why are metric sizes 2/3rds metric but the tire radius is in inches? Do they do it that way in Europe? I never heard of the yellow dot described in this article.
Forum Discussion
Wow, there was a lot wrong with that article.
First let's talk about the rim diameter in inches. Yes, almost since the time tires were invented, the rim diameter has been expressed in inches - but so were the rest of the dimensions. It's only been since the introduction of radial tires that metric dimensions have been used - and since it isn't easy to express rim diameter in anything other than inches, the practice was retained.
As was pointed out above, there have been several attempts to introduce metric rim sizing - the most notable, Michelin's TRX series - but inevitably, that was rejected by the marketplace.
The dots on the sidewall? Those are supposed to be for uniformity (think runout and you'll be close!), not for balance. The high point of the tire is supposed to be matched up with the low point of the wheel, so the assembly is rounder. Every car manufacturer does that, but the valve hole is only sometimes used to mark the low point of the wheel. - AND - In the replacement market, tires aren't always marked - and red is the more common color.
There's no harm done matching up the dots on the tire with the valve hole on the wheel, but most of the time, it doesn't do anything of value.
And let me state this again. This is about out-of-round, not balance!
And, Yes, the US regulations require the load carrying capacity be delineated on the sidewall. I can't remember if the regulations say both English and metric units are required, but it is customary to do so.
And lastly: Starting in 2015, the DOT code - aka as the TIN (Tire Identifcation Number) - can be up to 13 digits long (formerly 12). That's because a 3 digit plant code is now permissible (as opposed to the old 2 digit plant code.) In 2025, it becomes mandatory.
First let's talk about the rim diameter in inches. Yes, almost since the time tires were invented, the rim diameter has been expressed in inches - but so were the rest of the dimensions. It's only been since the introduction of radial tires that metric dimensions have been used - and since it isn't easy to express rim diameter in anything other than inches, the practice was retained.
As was pointed out above, there have been several attempts to introduce metric rim sizing - the most notable, Michelin's TRX series - but inevitably, that was rejected by the marketplace.
The dots on the sidewall? Those are supposed to be for uniformity (think runout and you'll be close!), not for balance. The high point of the tire is supposed to be matched up with the low point of the wheel, so the assembly is rounder. Every car manufacturer does that, but the valve hole is only sometimes used to mark the low point of the wheel. - AND - In the replacement market, tires aren't always marked - and red is the more common color.
There's no harm done matching up the dots on the tire with the valve hole on the wheel, but most of the time, it doesn't do anything of value.
And let me state this again. This is about out-of-round, not balance!
And, Yes, the US regulations require the load carrying capacity be delineated on the sidewall. I can't remember if the regulations say both English and metric units are required, but it is customary to do so.
And lastly: Starting in 2015, the DOT code - aka as the TIN (Tire Identifcation Number) - can be up to 13 digits long (formerly 12). That's because a 3 digit plant code is now permissible (as opposed to the old 2 digit plant code.) In 2025, it becomes mandatory.
