History Why Alphanumeric tires sizes?

For some time I've been curious about the history of tire sizing and construction. One thing I've never understood or found any specific history about is the adoption of U.S. alphanumeric sizing in the 1960s (it doesn't help that I was born in '83). It seems to have come out of nowhere. There was numeric sizing (e.g. 600-16; 670-15) up to and through the 1960s, and then all the sudden in the late 1960s there was the 70 and 78 series alphanumeric sizing (e.g. G70-14; H78-15, etc.). Many, if not all, U.S. tire manufactures adopted this nomenclature, so I assume there was some government regulation that forced it, but I don't know. I understand the that cars were changing and lower aspect ratios were being developed (so there was a need to develop a new system), but what is the history of this wholesale adoption of alphanumeric sizing?

 

https://www.mecum.com/articles/tire-size-101/#

 

https://www.tires-easy.com/blog/how-to-find-your-tire-size/
A Brief History of Tire Size Standards
Tire sizing standards emerged as the automotive industry grew, and the need for interchangeability and safety became apparent. Initially, tire sizing was not consistent, making replacing tires confusing for vehicle owners.
Over time, industry organizations established standards to create a common language for tire sizing:
The Tire and Rim Association (TRA) in the United States, established in 1903, sets forth guidelines for tire sizing and measurements.
The European Tyre and Rim Technical Organization (ETRTO), established in 1964, provides a similar standardization framework in Europe.
These standards have been adopted internationally and provide a consistent format for global tire sizing. Today’s sizing system is metric mainly, specifying tire width, aspect ratio, internal construction, and diameter of the rim that it fits.

This one by Coker goes into much greater detail, which should answer your question
https://www.cokertire.com/tire-size
The government can suggest or enforce things, but it's more common to have either manufacturer's agree as mentioned above, or a group like SAE try to get everyone to adapt standards. For example, they got everyone to agree that the DTCs were all the same for the P0000 thru P0699 and have transmission codes be P07XX, but everything P1XXX is manufacturer specific.

BTW, this is the smart way to go in most cases. Think about your local cruise night. It's better to self police within reason than to have the local cops be the first line of enforcement...

 

Things like tire size nomenclature usually are not the result of gov't...instead, they're the result of tire companies getting together to ruin our lives.

Change is good.

 

But buckets of large denomination unmarked bills is even better.

 

I'm old enough to be glad they haven't changed all wheels to be standard metric.
15" equals 38.1 cm.
How'd you like 40cm wheels thrown in the mix?

 

^^^^^ Isn’t metric bolt patterns enough?

 

Remember those stupid metric wheels in the '80s?
Some Fords ( and a couple European cars) had them, I think the wheel size was 390. Luckily a standard wheel bolt / stud pattern was retained so that conventional wheels could be fitted after the stock wheels were thrown away.

 

TRX ,shudder!

 

That's quite the opposite on the wheel diameters, metric was abandonned in favor of inches in the late 50's early 60's, width is metric then the aspect ratio is a percentage, so it doesn't matter if it's metric or imperial.
There are negociation between imperialistics and metrics to know which will be used in which industry, for example sheet metal screws are imperial all over the world but the dimension will be advertised in metric in all countries except for the 3 who are still living in the middle ages . This is why in metric sheet metal screws have weird diameters.

 

The Mecum article (and likely the Coker article as well) were written by Wade Kawasaki, CEO of Coker. He simply says that alphanumeric sizing became common in 1967. He references nothing and provides no supporting information or history. He’s probably right, as Coker has purchased many of the vintage molds, so I assume he has access to lots of tire company information. My question is why. Why the alphanumeric system? What is the specific history behind that nomenclature? If the TRA was involved….in what way? What hearings, meetings, or legislation, specifically occurred that caused the adoption of alphanumeric sizing?

 

I seem to remember reading a story about it when I was young, in a back issue of a Mechanix Illustrated type magazine. But that was 50 years ago, so I forgot!

 

It came about when the tire companies figured out how to make wide tires. Before that, all tires were as tall as they were wide. This is called a 100% aspect ratio.

Then they started making tires that were wider and lower. Like 78% aspect ratio or even lower. So they made up a new sizing system for the new tires consisting of a letter for the size, a number for the aspect ratio, and another number for the wheel rim size. So an H 78 15 tire was bigger than a G 78 15, but both would fit on a 15" wheel.

They also came out with 16.5 truck rims about the same time. They already had 16" truck rims but they wanted to stop guys putting the new wide tires on the old skinny rims. So they made a new rim 1/2" bigger. This lasted for about 20 years until all the old rims were off the road then quietly dropped.

When radial became popular they had to come up with yet another new system. Now you can get 70, 60, 50 and even lower.

 

The radial came out in 46 it was the Michelin X but it was invented by an american much earlier than that, the Citroën 2cv was the first new car equiped with these tires from the factory in 48.
The letter system as far as I know was never implemented in Europe, it seems it's purely a US market fad.

 

I recall older tires being an 82 series, then they went to 78. Sizes would be like a G78-15 which was essentially a 8.0 to 8.25 tire in the old system.

The odd size truck tires indicated a tubeless designation, I have had old trucks with 19.5 and 17.5 wheels on them. The 16.5 came around a little later. For trailers, the 14.5 was very popular, it also made sure that car tires couldn't be used on them.

 

U-haul used the 14.5 and other oddball sizes, so a renter couldn’t think “man, I’ll swap these tires for the trailer I just rented”.
I recall a trailer I rented once and it had 14.5’s on it. One was low, noticed the size when I put air it.
When I returned the trailer, I let the guy know it may have a leak, the asked him why the odd size.
He told me “ so you can’t use them”.

 

Interesting perception. Can't say I've noticed it down here.

 

The alphanumeric tires were based on the load weight range the tires were designed to carry. The older number base tires were strictly based on the physical tire size. The modern metric sizes are also physical tire size based, but also include a load weight range.

I no longer remember what load range each letter covered, but generally a load range could cover a 200- 300 pound range, and the next letter was the next higher load range. By going with the load range, the wheel diameter, the aspect ratio, or the tire being bias ply or radial did not enter into the weight load range. A "G 78 x 14" tire had the same weight range as a "G 60 R 15" tire had. Tires not capable of caring the vehicles weight could easily be determined, but over sized tires were not a concern. Often the next letter size up from the standard equipment tire sizes were the next level optional size tires. If tires with an "F" load range was the standard equipment tires, a "G" load weight range would be the next larger optional tire size.

The tire load range tires started with a "C" load weight range. Then they proceeded through the alphabet skipping only an "I" weight range, and a "K" weight load range. A tire caring a "N" weight range was the largest letter size, but I've never seen the load range weight of an "M" or an "N". Both of those were primarily very large performance tires.

Tires with a "C" load weight range were often the compact car standard equipment tires.

Tires with an "F" load weight range were often the performance era intermediate sedan standard equipment tires.

Tires with a "G" load range were often light duty 1/2 ton pickup truck standard equipment tires.

Tires with an "H" load range were often the large luxury car standard equipment tires.

 

I remember back in the late 70’s -early 80’s a N50-15 was about the biggest street tire you could get. Light truck tires were still using the numerical system for their larger tires, 10.00 30 x15, 12.50 33 x15, etc. Now they use the metric systems, and it makes it harder for me to figure out what I want. My pickup had 265 70 x 17 on it, with a little research I found a 265 65 x17 had the same installed height, with only 1/8” difference in width, and were $25 cheaper per tire! Come to find out when mounted, they were exactly the same size width and height! Now these were the same brand and style tire, there would probably be a difference between brands, which if they were going strictly by a size standard they should be exactly the same.

 

One thing I've learned is those 'tire size conversion' charts have to be taken with a grain of salt. One issue is due to improvements in tire construction, load ratings have gone up in comparison to actual physical size. The other major issue is aspect ratios. While ratios in the 85 to 90 ranges were common in the '50s, a 75 ratio is now the 'tallest skinny' size generally available unless you go to a specialty 'vintage' design tire. The charts attempt to balance this between the physical diameter/width and the load, but most seem to apply the most weight to the load rating which can result in tires that look small on your older car.

 

Just let yer L 60-15s hang out da fender lip a bit and it's good to go...

 

265 is the width in mm so both tires have the same width. The aspect ratio is 5% smaller which in this case should be a 25.5mm or just 1" in the tire overall diameter, they shouldn't come up with the same height.

 

One thing I noticed is that the 75 aspect ratio tends to be a US thing. Outside the US you'd find 70 and 80 but no 75.

 

I think you are doing it wrong...

@chrisp is correct. I get a difference of 26.5mm in diameter, a little over 1".

 

In a perfect world that would be right... but not when it comes to tires sizes. Spend some time comparing actual sizes to advertised sizes, there can be some pretty big differences. Both brand and even tire lines within a brand can vary quite a bit for the 'same' tire. Some will run big, some small. In an OEM application these differences probably don't matter, only if you're trying the 'fill the wheel well' with an oversize does it start to get critical.

 

Tire size is an interesting dilemma.
Back in my dirt track days (mid 70s) our class had tire restrictions. We were suppose to run the same size tires all around the car. I worked at a gas station and mounted a lot of tires. I knew some brands regularly produced tires that were taller or wider then the same size tire from other brands. I also knew that when you inflated a new tire for the 1st time, if you ran the tire pressure up as you "set the bead", that tire would be taller, and have a larger width then if you could 'set the bead" with low pressure.

We could have all 4 tires of the same size, with 4 different brands, and with 4 different "set the bead" pressures that had 4 different outer diameters and 4 different tread widths. I could set my cars stagger by simply knowing which tire to put in what position on the car. When they told me I had to run all the same brand of tire, I replied OK. The width of the wheel made a difference as well. I still had no problem setting the stager on the car.
Tire size is just a starting point.

 

On one of my old cars the cheap spare tire just fits in the well in the trunk, but a Michelin of the "same" size will not.

 

It is ironic that the French Monarchy supported the US in its revolution, and a couple years later met a bad end in France's own revolution.

 

lol,,,,,yeah,,,,Charles de Gaulle was a top rate guy,,,,nearly cost us the landing on Normandy with his waiting to endorse the program !

Sorry,,,I wasn’t speaking directly about the French when I made my post before .

However,,,,I DON’T back up one word from what I said !
There are a lot of graves near Normandy with the last final measure of our boys to prove it .

Tommy

 

you're probably still using 1/4, 3/8, and 1/2 inch ratchet handles. Just a touch of imperialism to keep the commies away...

-rick