BFG KM2 True Sizing

BuildBreakRepeat

BuildBreakRepeat

Occasional Genius
Supporting Member
Joined
Jan 21, 2019
Messages
3,371
Location
McKenzie Bridge, Oregon
Information update for this that are interested. As many of you know, BFG runs their tire sizes small (one of the many reasons I’m switching). After 30,000 miles, my 31575R16 KM2s measure 32” with 26psi in them. Free mounted (spare tire) they measure 33”. 31575R16 is equivalent to 34.6” - and these numbers are a little disappointing to me. Anyway, just wanted to add more technical information on these tires and answer any sizing questions.

B80B7A7B-1007-4EB8-BAAB-2FA4962DC436.jpeg

(Old picture without Antirock or wide flares)
Can’t consider yourself a true Oregon jeeper until you’re in the snow without doors ;)
 
Last edited:
  • Like
Reactions: psrivats
@Starrs and I can both attest first hand that both of our brand new KO2s mounted on the spare tire with 100% tread measured exactly 33.2" (this was for a 315"), which is drastically under the 34.4" they claim.

So in other words, a BFG KO2 in a 315 size is really a "true" 33".
 
  • Like
Reactions: Starrs and BuildBreakRepeat
bobthetj03 said:
From what I've researched, General Tire makes a more true size. My KO2's are anemic.
Click to expand...

Chris said:
@Starrs and I can both attest first hand that both of our brand new KO2s mounted on the spare tire with 100% tread measured exactly 33.2" (this was for a 315"), which is drastically under the 34.4" they claim.

So in other words, a BFG KO2 in a 315 size is really a "true" 33".
Click to expand...

Almost everywhere I’ve looked BFG have this complaint. Almost all other brands apparently get it right - I was under the impression there was a national standard for how to measure (max pressure on specific rim size with no load) so I’m confused how they can advertise a tire much larger and sell it for an insane price.
 
  • Like
Reactions: HardSell and Chris
BuildBreakRepeat said:
Almost everywhere I’ve looked BFG have this complaint. Almost all other brands apparently get it right - I was under the impression there was a national standard for how to measure (max pressure on specific rim size with no load) so I’m confused how they can advertise a tire much larger and sell it for an insane price.
Click to expand...

Yep, I wish I had done the research. It doesn't change the fact that they make really good tires, but the sizing thing pisses me off. I wanted the 315 because I figured at 34.4", it would be as close as I could get to a 35" while still maintaining the C load rating (the true 35" was E rated).

Of course as we both found out, it's nowhere near 34.4". I tell people now that if they want a BFG tire in a true 35, they need to buy a 37" size!
 
  • Like
Reactions: JMT, Starrs and BuildBreakRepeat
Does anyone know if the KM3's are the same situation? Looking at getting some 33x10.50r15's but maybe not if they end up being the same size at my current 31x10.50r15's lol
 
Hammer24 said:
Does anyone know if the KM3's are the same situation? Looking at getting some 33x10.50r15's but maybe not if they end up being the same size at my current 31x10.50r15's lol
Click to expand...
Starrs said:
No experience with the km3 yet, but I’d venture to say it’s the same song and dance. Go for the 315 and you’ll get a 33 for sure lol
Click to expand...

my 33x12.5 KM3s are at best 32, I've measured 16 inches to the center of the axle tube as best as I can tell, I haven't figured out a better way to accurately measure them. 35s are probably also shorter.

Violently effective tires though, whatever size they are.
 
  • Like
Reactions: HardSell and JMT
Westtown Willy said:
my 33x12.5 KM3s are at best 32, I've measured 16 inches to the center of the axle tube as best as I can tell, I haven't figured out a better way to accurately measure them. 35s are probably also shorter.

Violently effective tires though, whatever size they are.
Click to expand...
I have zero complaints with the tire itself, works just how I would want them to. I’m sure if I measured from center to top it would be larger, my measurement was to show the height from ground up including the weight. I’ve heard good things about KM3s, I’m sure they are just an improvement on mine.
 
all BFG's run small? my brakes and 1 ton steering force me to have a 17" wheel to get over it all. the only wheel i find class C is the BFG MTR.
there are a few D's (MT's), but C is almost nonexistent at 17".

anybody know if the MTR"s run small too? i was looking for a 35x12.5, are those gonna be truer to size than the 315's?
 
  • Like
Reactions: BuildBreakRepeat
I run 33 km3s on my SE with 4.88s,and one of the reasons I got them is I knew bfg runs on the smaller side so I was aok with that. They are just a smidge above 32" on mine at 28psi on 15x8 wheels.

They work great for me btw,and dig down and hook into traction pretty good in the slop.
 
  • Like
Reactions: BuildBreakRepeat
someguysjeep said:
all BFG's run small? my brakes and 1 ton steering force me to have a 17" wheel to get over it all. the only wheel i find class C is the BFG MTR.
there are a few D's (MT's), but C is almost nonexistent at 17".

anybody know if the MTR"s run small too? i was looking for a 35x12.5, are those gonna be truer to size than the 315's?
Click to expand...
I’m pretty sure MTRs run true. The 37” BFG KM3 is almost exactly a 35” tire. So if you’re okay with D they make it on a 17. Why not go with Nitto trail Grapplers? 35x11.5 on a 17” rim load range C
 
I’ve been very happy with the performance of my KO2s so far, but will probably go elsewhere next set to get the sizing better. That decision is a few years away though with how little I drive!
 
How are those of you who claim BFG's "run small" measuring your tires?

If from the top of the tire to the ground when inflated, mounted on the vehicle, and on the ground your measurement will reflect the loaded radius and differ substantially from published tire sizes.

If from the hub to the ground then multiplied x2 your measurement will also reflect the loaded radius and differ substantially from published tire sizes.

If measured horizontally at the hub with the tire mounted, inflated and on the ground your measurement will still differ from published tire sizes, but less so than the other methods.

The industry standard is to measure with the tire mounted, inflated and off the ground with no load. If one runs a same size spare one can measure the diameter of the spare tire rather than a tire on the ground, multiply the radius of the spare x2, or better yet measure the circumference of the spare with a cloth tape then divide by 3.14

When one has measured the same way as the industry standard then one can state with some authority that BFG's "run small."

But smaller than what standard? Tire manufacturers never claim that their 33's are exactly 33" in diameter when mounted and inflated, or that 35's are 35", 37's are 37", etc.

Industry standard actual sizes compared to sidewall markings:

33x12.50R15 - 33" (+/- 0.50")
35x12.50R15 - 35" (+/- 0.50")
285/75R16 ("metric 33") - 32.8"
315/75R16 ("metric 35") - 34.6"


BFG published diameters for KM3's:

33x12.50R15-C - 32.5" when inflated to 35 psi on a 10" rim
35x12.50R15-C - 34.5" when inflated to 35 psi on a 10" rim
285/75R16-E ("metric 33") - 32.8" when inflated to 80 psi on a 8" rim
315/75R16-E ("metric 35") - 34.6" when inflated to 65 psi on a 8.5" rim.


Goodyear published diameters for the MTR:

33x12.50R15-C - 32.8" when inflated to 35 psi on a 10" rim
35x12.50R15-C - 34.8" when inflated to 35 psi on a 10" rim
285/75R16-E ("metric 33") - 33.1" when inflated to 80 psi on a 8" rim
315/75R16-D ("metric 35") - 34.8" when inflated to 50 psi on a 8.5" rim.

BFG KO3's actually run about the same as the industry standard, not smaller. GY MTR metric sizes actually run slightly larger than the industry standard, and on average about 0.3" larger than the corresponding size BFG KO3 with the same mileage, but good luck trying to measure that difference in the real world. After one gets some miles on a set of the tires any difference in the published diameter of the brand purchased from the published diameter of the brand not purchased becomes relatively meaningless - and even more difficult to compare if there are also differences in the published tread depth.
 
Last edited:
  • Like
Reactions: HardSell, JMT, bobthetj03 and 1 other person
Mr. Bills said:
How are those of you who claim BFG's "run small" measuring your tires?

If from the top of the tire to the ground when inflated, mounted on the vehicle, and on the ground your measurement will be inaccurate

If from the hub to the ground then multiplied x2 your measurement will be also be inaccurate.

If measured horizontally at the hub with the tire mounted, inflated and on the ground your measurement will still differ from published tire sizes, but less so than the other methods.

The industry standard is to measure with the tire mounted, inflated and off the ground with no load. One can measure the diameter, multiply the radius x2, or better yet measure the circumference then divide by 3.14

When you have measured the same way as the industry standard then you can come back and state with some authority that BFG's "run small." Until then you are just reporting your own measuring discrepancies.

Also, tire manufacturers never claim that their 33's are exactly 33" in diameter when mounted and inflated, or that 35's are 35", 37's are 37", etc.

Industry standard actual sizes compared to sidewall markings:

33x12.50R15 - 33" (+/- 0.50")
35x12.50R15 - 35" (+/- 0.50")
285/75R16 ("metric 33") - 32.8"
315/75R16 ("metric 35") - 34.6"


BFG published diameters for KM3's:

33x12.50R15-C - 32.5" when inflated to 35 psi on a 10" rim
35x12.50R15-C - 34.5" when inflated to 35 psi on a 10" rim
285/75R16-E ("metric 33") - 32.8" when inflated to 80 psi on a 8" rim
315/75R16-E ("metric 35") - 34.6" when inflated to 65 psi on a 8.5" rim.


Goodyear published diameters for the MTR:

33x12.50R15-C - 32.8" when inflated to 35 psi on a 10" rim
35x12.50R15-C - 34.8" when inflated to 35 psi on a 10" rim
285/75R16-E ("metric 33") - 33.1" when inflated to 80 psi on a 8" rim
315/75R16-D ("metric 35") - 34.8" when inflated to 50 psi on a 8.5" rim.

BFG KO3's actually run about the same as the industry standard, not smaller. GY MTR metric sizes actually run slightly larger than the industry standard, and on average about 0.3" larger than the corresponding size BFG KO3 with the same mileage/age, but good luck trying to measure that difference in the real world. After one gets some miles on the tires the differences in the published sizes are relatively meaningless.
Click to expand...
I’ll measure when I get home.
 
Mr. Bills said:
How are those of you who claim BFG's "run small" measuring your tires?

If from the top of the tire to the ground when inflated, mounted on the vehicle, and on the ground your measurement will be inaccurate

If from the hub to the ground then multiplied x2 your measurement will be also be inaccurate.

If measured horizontally at the hub with the tire mounted, inflated and on the ground your measurement will still differ from published tire sizes, but less so than the other methods.

The industry standard is to measure with the tire mounted, inflated and off the ground with no load. One can measure the diameter, multiply the radius x2, or better yet measure the circumference then divide by 3.14

When you have measured the same way as the industry standard then you can come back and state with some authority that BFG's "run small." Until then you are just reporting your own measuring discrepancies.

Also, tire manufacturers never claim that their 33's are exactly 33" in diameter when mounted and inflated, or that 35's are 35", 37's are 37", etc.

Industry standard actual sizes compared to sidewall markings:

33x12.50R15 - 33" (+/- 0.50")
35x12.50R15 - 35" (+/- 0.50")
285/75R16 ("metric 33") - 32.8"
315/75R16 ("metric 35") - 34.6"


BFG published diameters for KM3's:

33x12.50R15-C - 32.5" when inflated to 35 psi on a 10" rim
35x12.50R15-C - 34.5" when inflated to 35 psi on a 10" rim
285/75R16-E ("metric 33") - 32.8" when inflated to 80 psi on a 8" rim
315/75R16-E ("metric 35") - 34.6" when inflated to 65 psi on a 8.5" rim.


Goodyear published diameters for the MTR:

33x12.50R15-C - 32.8" when inflated to 35 psi on a 10" rim
35x12.50R15-C - 34.8" when inflated to 35 psi on a 10" rim
285/75R16-E ("metric 33") - 33.1" when inflated to 80 psi on a 8" rim
315/75R16-D ("metric 35") - 34.8" when inflated to 50 psi on a 8.5" rim.

BFG KO3's actually run about the same as the industry standard, not smaller. GY MTR metric sizes actually run slightly larger than the industry standard, and on average about 0.3" larger than the corresponding size BFG KO3 with the same mileage/age, but good luck trying to measure that difference in the real world. After one gets some miles on the tires the differences in the published sizes are relatively meaningless.
Click to expand...

It may not be all posts, but a good number comment on measuring free mounted on the tailgate. Maybe not to 80psi though, yikes.
 
When shopping for tires, look up the "revs per mile" in the tire spec section.
Take 20168/revs per mile, this will give you a tire diameter in inches.

This method will give you a better idea of the actual tire diameter, however, it doesn't account for tire inflation pressure or rim width.

Here is an example with 315/75/r16 BFG KO2's
Screenshot_20200211-173627.png


20168/602=33.5
That's only 33.5" for a 35" nominal size.


You can use this to compare tires while shopping without actually seeing them in person. Hopefully this helps.

Edit: notice how BFG claims this is a 34.5" tire
 
  • Wow
  • Like
Reactions: JMT and BuildBreakRepeat
Top Bottom
Back