The Official TIRE SIZE THREAD

 

 

On my Montego wagon

 

Fronts are 17x8 w/ 4.5" BS on 235-55 tires

Rears are 17x9 w/ 5.25" BS on 275-55 tires

 

 

 

 

 

I assume you mean aspect ratio but would you explain why you say this please. 

 

Actually not what I was thinking but you're right. kudos

 

It's been years since I've considered this so maybe it's just "old school" nonsense today on my part but I don't see them as the same.

 

Aspect ratio was a static number based on the off the rim section height and width. It doesn't take into account rim size. With the proliferation of low profile tires anyone using the number needs to ensure apples and oranges aren't getting mixed.

 

Profile ratio was an entirely different animal. It's an active number based on a tires limits and used primarily by but not limited to engineers for matching a tire to a vehicle or in motorsports, conditions. It's use is necessitated by Newton's 3rd law of motion, "for every action there is an equal and opposite reaction." Can a tire handle the performance characteristics of a given vehicle or can a vehicle handle the performance characteristics of a tire. Look at the tire rotation directions through the years. Tires today may have the same inside outside profile ratios but they are not the same left to right. Don't put a right tire on the left side. The sticky tires used in F# racing would tear up a daily driver's drivetrain and there's a good reason why biased tires are almost a thing of the past. Consider a tire used by an airline where the drivetrain is separate from the tires. These tires are designed to get taller and thinner as they approach their "at speed" limit. The approach to traction between an auto and airline is very different. A tire has more then one profile ratio because it's an "in use" number. In my day anyway.

 

In my day the equivalent and effective ratios were what mattered. Ensuring your tires matched the rear end gearing. Those cool, big tires often came at the expense of performance.

 

Don

 

Barnaby,

 

You're absolutely right. For road vehicles all the ratings come from or are adopted from the SAE. For the daily drivers it's not much of an issue anymore because of the research that has been done and the ratings. Sometimes the envelope gets pushed though. I can't say where I heard or read this but for the newest Veyron there is only one legal tire for the car at it's rated top speed. Even then the life of the tire at speed is rated in either minutes or miles at a cost of $30,000 for a set. I just looked that cost up. You can bet tire companies are working on tires set for a new rating to accommodate the car.

 

The ratings exist for motorsports also. I'm not sure how familiar Brit's are with tractor pulling but when pulling started using engines like the Allison Merlin there were problems with the split rims on tractors and tires. That was solved by new ratings. Jim Thorton who in the 60's invented the drag racing torque cancelling chassis, is credited as the inventor of funny cars, was the head of Chrysler's race vehicle design and the primary driver of Chrysler's drag racing team talked a lot about how tire profile didn't keep up with the sport. Nitro burners were around long before they were legal on the strip because of tires. Even today the tires require being screwed to the rim.

 

The ratings per tire are based on profile ratios which is about the tire in use and not a static, off the rim ratio of section height and width. And it is section height, not tire radius.

 

"It's an adjustable wrench, Crescent is a brand." I heard this all the time growing up. I'm aware of how words and meanings change because of common use. I'm not at all being critical of you. Some things do irritate me but this isn't one of them. I'm only explaining what the meanings are for me.  

 

Don