Tire Height vs. Inflation (1 Viewer)

[WiscoProud]

Quick question before I dig in: When a manufacturer lists the height of a tire, what pressure (if any) is that measured at?
I have a set of General Grabber AT2 in LT275/70R18 on my 100. I keep them at 45-50psi to help with fuel economy. I was screwing around with a tape measure the other day and realized that on the truck they only measure about 32", rather than the 33.2" listed by the manufacturer. I know the weight of the vehicle and pressure will affect the diameter, but I didn't think I would lose 1.2", and especially not while running a relatively high level of pressure. The max tire pressure is 80psi
I have searched high and low here and on the interwebs looking for an explanation, to no avail.
Could one of you measure the height of your tires for comparison purposes?
Thanks!

 

[TXLX100]

Sounds pretty standard. They might be measuring diameter, not mounted on vehicle and at max inflation. If not, might be measured not mounted on wheel.

My 32.8" tires are closer to 32" on the vehicle.

 

[hoser]

The diameter should really be measured horizontally or unloaded. The height measurement that takes into account the load and pressure would be the "static loaded radius." Some manufacturers give this specification (along with the measured load and tire pressure).

If you are trying to make calculations, the tire's overall diameter also changes according to speed... but I think at freeway speeds, the measurement will be pretty close to the tire's true unloaded diameter.

 

[WiscoProud]

I hadn't heard of the "static load radius" measurement before, that appears to explain my difference. However, I can't seem to find any information regarding my vehicle for it.

Logically, it would seem that if your tire height is measuring short loaded compared to unloaded, you may need to add air, as the tire is compressing under the weight of the vehicle.

Thoughts?

 

[Mace]

Tire MFG's numbers are based on a specific rim and pressure (varied per tire) And, they lie a lot..

 

[WiscoProud]

So I found something that indicated the "revolutions per mile" spec takes static load radius into consideration. So I took the stated revs and backed into the diameter they're using for it, which came out to approximately 32.2". So while my tires are slightly lower than spec, they're not far off. I'll just try putting in another 5-10 pounds to see if the difference is made up.