True Tire Heights? - Lateral-g Forums

OLDFLM · #1 05-15-2008, 09:12 AM

I'm looking for new wheels/tires for my 71 Firebird and for reference I took someone's suggestion and went out to measure my fender lip heights. With the current rolling stock of 255/60/15s and 275/60/15s the front fender lip measures in at 27.5" with the 255/50/15 at 26" and the rear fender lip measures in at 28.5" with the 275/60/15 at 27".

My question is that the 275/60/15 supposedly measures 28" according to most charts not the measured 27"... what's the deal? The tires are mounted on 15x7 Pontiac Rally IIs. When looking at new tires, say a 285/40/18 for out back, the charts say they measure 27", but do they really?

V/R,
Ty

astroracer · #2 05-15-2008, 10:36 AM

Tire diameter is dependent on rim width. Your 7" rim is making the tire taller. I believe the recommended rim width for a 275 is 8"-10" so your narrow rim is creating a taller profile.
When checking tire sizes you will see that the diameter is measured on a certain width rim. Going wider or narrower will alter the number slightly and an inch isn't uncommon.
Mark

OLDFLM · #3 05-15-2008, 10:47 AM

Normally I'd agree... but it's actually measuring an inch shorter than advertised...even on a 7" rim! ???

79ZED · #4 05-15-2008, 08:54 PM

Tire diameter is not the rolling diameter. What you need is the rolling radius of the tire at typical inflation pressures and load weights. Tire manufacturers usually have this info.

John

LCAC_Man · #5 05-16-2008, 09:02 AM

Quote:

Originally Posted by OLDFLM

Normally I'd agree... but it's actually measuring an inch shorter than advertised...even on a 7" rim! ???

Good luck getting this from any of the tire manufacturers. The advertised specs always list the tire height as more that it truely is. Very few are closer than an 1" of the true height.

72blackbird · #6 05-21-2008, 10:30 AM

I checked the 235/60R15s on 8.5" rims currently on my car now- they measure 25.25", not the 26" spec listed in BFG's literature. As ZED mentioned, the rolling diameter increases due to heat generated when the tire is in use.

Geno

gearheads78 · #7 05-21-2008, 01:11 PM

You are measuring with the weight on the car on it. Its much closer to advertised if you measure off the car but aired up and mounted on a wheel.

Silver69Camaro · #8 05-21-2008, 02:44 PM

Quote:

Originally Posted by gearheads78

You are measuring with the weight on the car on it. Its much closer to advertised if you measure off the car but aired up and mounted on a wheel.

Is that what you guys are doing? General rule of thumb is the rolling radius is 5/8" to 1" shorter than unloaded radius. Rear is about 1/2".