Which tire brands are true to size?

[SpaceRat]

I have just learned that most tires are not true to size and technically may be smaller than the marketed size. Meaning you buy a 35" tire, but in all reality, it's a 34.5" tire.

So which tire brands are true to size? If you buy a 35" tire, you should get a 35" tire.

 

[smritte]

Heh.
Welcome to the world of tires.
You can easily calculate tires diameter by their posted sizing. 305/75 17.
Tread 305mm across
Sidewall 75% of 305mm
17 inch rim.
Formula

305 X .75 = 228.75 This is the height of one sidewall
228.75 X 2 = 457.5mm. this is the top and bottom sidewalls combined without the rim in the center.
457.5 / 25.4= 18.01 inch. This is to change mm to inch. rim is measured in inch.
18.01 + 17 = 35.01. Need to add in the hole where the rim goes.

35 inch tall tire.
See Easy

Except. When they measure the tread width (305mm) where exactly do they measure from? This is the main issue. If they measure slightly towards the sidewall, this makes the formula read high.

They will also give the measurement with one rim width (8", 7") and no weight on the tire.
Lets say they advertise this as a 36" tire on a 7"wide rim. You put it on a 10" rim and it will be shorter. You put it on your vehicle and it squish's down some, shorter again.

All of the tires I have measured are typically 1/2" shorter, on the proper rim size. If I want a 33" tire, I look at the manufacture specs, including recommended rim size and subtract 1/2 inch.

Here's a fun thing to do. Take your tire width (305mm here) and cut a string to that length and pull it across the tread. Now try to guess how they came up with this measurement.
That is actually a Lab I would give students to teach this exact thing. I send them into the parking lot with a tape measure and string. Its easier to do this with a piece of string then it is to just use the tape measure.

 

[smritte]

Here is an online calculator that I use. You can put stock vs new or just put the same numbers in to see what the size is. This is more designed for comparison.
Calculator

Its great for comparing tires but, will give the calculated size, not actual. I subtract 1/2" from what it shows.
Another nice thing is, if your going to a wider tire, it will show the difference in width. If you measure how much clearance you have turning, you can get an idea if the new tires will rub. It also tells you the speedo error with different tire diameters.

 

[Randy]

Tire manufacturers evaluate tire dimensions on a particular wheel width, which also accounts for the protrusion on the sidewall. Therefore, the dimensions you find on their "specifications" pages ARE reliable.

However, when you or I carry out the measurement, we use a direct vertical line from the ground, plus the weight of the vehicle is exerting pressure on the tire. This method inevitably leads to a smaller measurement.


I've utilized countless AT and MT tires in the past ~25 years. My observation is that the majority of brands have extremely similar sizes when you compare similar models.