Can someone please explain how to measure the diameter of a tire? Ive had 3 sets of MT's so far 31x10.50x15....none of them have been bigger than 30.5.....and now i just got a set of TrXus MT's same size....properly inflated, and i'm measuring from outside edge to outside edge and i'm coming up w/ 30.00"...whats the deal....everyone keeps saying the TrXus is one of the few 31's that are actually 31's. What am i doin wrong. Thanks for any info.
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Actual tire diameter??
- Thread starter JeepXJ93
- Start date
scorpio_vette
NAXJA Forum User
- Location
- Milwaukee Wisconsin
your not doing anything wrong. i don't know why they are different than advertised. maybe somebody else can chime in on that.
but for future reference, if you want to know exactly how tall they will be before you buy them, do some research on the manufacturers website. they usually list all the actual tire measurements.
like this for example:
http://www.intercotire.com/site29.php
but for future reference, if you want to know exactly how tall they will be before you buy them, do some research on the manufacturers website. they usually list all the actual tire measurements.
like this for example:
http://www.intercotire.com/site29.php
MyJeepXJ
NAXJA Forum User
- Location
- SW Virginia
Want to have more fun? Measure a 2"X4"! Its only accurate in LENGTH. 
MyJeepXJ
NAXJA Forum User
- Location
- SW Virginia
Theres only 1 or 2 that make tires that are closer to the "actual" dimentions. Thats about all I can say. And no, its not a major name brand manufacturer, like Goodyear or BFG, they build the same as cheap'os.
I take it you are measuring them while on the XJ and with the XJ on the ground, correct? There's the problem. Even when a tire is supposed to be the stated size, it won't be when the vehicle's weight is on it.
If my assumption about being mounted with the XJ's weight on it is wrong, I have no clue.
If my assumption about being mounted with the XJ's weight on it is wrong, I have no clue.
a_hiatt
NAXJA Forum User
- Location
- effingham, IL
just the way it is, deal w/ it. if you want real 33's get 35's. on the bright side it's just about the same for every tire so you should have an idea of what a "33" should look like on your rig.
as far as the 2x4 thing, im pretty sure 2" and 4" are the dimensions before the final planeing(sp?) or sanding
as far as the 2x4 thing, im pretty sure 2" and 4" are the dimensions before the final planeing(sp?) or sanding
flexjay87
NAXJA Forum User
- Location
- iowa city iowa
advertised diameter is almost always wrong, weather too short or too tall. i am sure it has to do with pin heads in marketing and or advertising. ithink all these companies have an agreement to go with 1 inch increments weather correct or not. if you call them and ask them what the actual measurement is they won't tell you either. bfg's are close, swampers run big , eveyone else is a crap shoot. Scorpio is correct about some websites though, they do list actual sizes sometimes.
My BFG 235/75/15 ATs measured out to 29" just like advertised (when new). The catch was that to measure that, they had to be mounted on a 15x7 rim, but not mounted on the vehicle, and inflated to the maximum listed pressure of 50 lbs. If those factors are considered, BFG is telling the absolute truth about that tire size's actual diameter. Real world, the tires work out to 27.75" at 32 p.s.i.
Starscream
NAXJA Forum User
- Location
- Columbia, SC
That's what I was going to say. Most manufacturer specs are based on max inflation on a more narrow rim. My ProComp 31s are actually more like 29s or so because I run them at about 30lbs of air. The max inflation is about 50lb I think, but they would ride like crap if I had them pumped all the way up.Timber said:
- Location
- Billings MT/Rapid City SD
Yeah I feel ya, my 31 parnelli's measured 31.25" tall and my 33's measured 33.25" tall. oh wait...
Ray H
NAXJA Forum User
- Location
- Falling Waters, WV
If you want to accurately measure dia. you find the circumference first.
Cir/3.14=dia.
The common wisdom is that the tire dia. will increase with more air pressure. Ive read some recent articles that dispute that because the construction of the modern tire doesnt allow the tread surface to stretch so the diameter stays roughly the same no matter what the pressure or what width rim its mounted on. Anyone whos seen a dragster come off the line would have a hard time believing that though. Maybe its just the difference in construction. I havent tested this so Im not saying one way or the other but its a good point for discussion.
Cir/3.14=dia.
The common wisdom is that the tire dia. will increase with more air pressure. Ive read some recent articles that dispute that because the construction of the modern tire doesnt allow the tread surface to stretch so the diameter stays roughly the same no matter what the pressure or what width rim its mounted on. Anyone whos seen a dragster come off the line would have a hard time believing that though. Maybe its just the difference in construction. I havent tested this so Im not saying one way or the other but its a good point for discussion.
Starscream
NAXJA Forum User
- Location
- Columbia, SC
I read this somewhere, but if you want to actually measure your rolling circumference, then you measure from the ground to the center of the hub and multiply by two. If you tires are a lil flatter at the bottom, which most are, you will actually have a smaller height while moving (but that can also be effected by speed as said above).Ray H said:
- Location
- Port Orchard, WA
Starscream918 said:
fixed
Starscream
NAXJA Forum User
- Location
- Columbia, SC
Oops. Thanks.XJ_ranger said:
