Tire size disparities

SSinGA

Jeep Fanatic
#1
It’s a well known fact that the BFG tires run small but how small are they really.

Here are a few pics of a 315/70-17 Cooper STTPro on a stock Rubicon Wheel (17x7.5}next to a 315/70-17 BFG KO2 on an AEV Wheel (17x8.5) and. 37x12.5-17 BFG KO2 on Raceline wheels (17x9). Viewers should note that the 315 BFGs have 50k miles on them at 33psi and are mounted under weight of a Light Build JKU. The 37” BFGs have 20k miles on them at 28psi and are on a heavy build JK. The STTPro is brand new not installed on a Jeep at 30Psi.

The 2 315s


315 and 37


What are y’alls experiences with tires that run big or small?
 
Last edited:
#2
I have never compared my 35" BFG KM2's against another 35 from another manufacture.
I have heard that the BFG's run smaller though.

Might be an interesting question to the BFG guys on why this is.

Todd
 

Jegar

Jeep Owner
#4
My new 35” BFG KM2’s measured a little over 33-1/4” mounted (17” Willys Wheeler rims) — 2dr with the stock plastic bumpers & fenders at the time :(


Sent from my iPhone using Tapatalk
 

Pthorpe84

Moderator
Staff member
#5
I haven’t compared mine either. But measuring my Toyo’s they run very close to the advertised size.


Sent from my iPhone using Tapatalk
 

SSinGA

Jeep Fanatic
#6
I haven’t compared mine either. But measuring my Toyo’s they run very close to the advertised size.


Sent from my iPhone using Tapatalk
The Toyo tires are closer in size very similar to the STT Pro and Maxxis RAZR.

The 35” Toyo Open Country MT is listed at 596 revolutions per mile making it a 33.8 under load.

The 37” OC MT is listed at 565 revs per mile making it a 35.7 under load.
 

Pthorpe84

Moderator
Staff member
#7
The 35” Toyo Open Country MT is listed at 596 revolutions per mile making it a 33.8 under load.

The 37” OC MT is listed at 565 revs per mile making it a 35.7 under load.
Interesting. Never looked at the data that way.


Sent from my iPhone using Tapatalk
 
#10
Yesterday I replaced my 315/70-17 Cooper STT Pro's with Falken Wildpeak 37x12.5x17 .. it's a bit of an apples to oranges comparison but the new Falken should not be any more than 2" larger than the coopers..but i would say they are a bit more than that.
 

Attachments

Last edited:

SSinGA

Jeep Fanatic
#11
Yesterday I replaced my 315/70-17 Cooper STT Pro's with Falken Wildpeak 37x12.5x17 .. it's a bit of an apples to oranges comparison but the new Falken should not be any more than 2" larger than the coopers..but i would say they are a bit more than that.
According to the Revs per mile number of 577 that 37 should be 34.95” under load. The 315/70-17 is 601 revs per mile so under load it is 33.55
 
Last edited:
#12
According to the Revs per mile number of 577 that 37 should be 34.95” under load. The 315/70-17 is 601 revs per mile so under load it is 33.55
I have no idea what a revs per pile number is but since I have to measure the tires to reprogram the ECU, i happen to have the actual measurements:
Under load the 315/70-17 was 33.75"
The new 37 is 35.5"

Once mounted and under load, the 37 does not look as giant next to an old 35..
 
#13
I have no idea what a revs per pile number is but since I have to measure the tires to reprogram the ECU, i happen to have the actual measurements:
Under load the 315/70-17 was 33.75"
The new 37 is 35.5"
You want to measure the loaded radius of the tire at normal street pressure, not the overall loaded diameter. Typically you can subtract 1/4-inch from the loaded diameter to get a reasonably accurate radius if you run relatively high pressure (>30psi) on the street. If you like to run a lower pressure, subtract 1/2-inch from the loaded diameter instead.

Or you can use a GPS to measure your actual highway speed, and then make corrections to your speedo by trial and error until you've gotten it dialed in.

I found that my 315/70R17 Cooper STT Maxx tires to be ~1/2-inch taller than my 315/70R17 BFG A/T-KO's were.

My next set of tires will likely be Milestar Patagonias. I'm still debating between 37x12.5R17 and 38x13.5R17 because I'm not sure how true to size they run, or how they compare to more well-known tires. I'm also trying to find a set of wheels I like that also have little enough backspacing (4.5" or less) for tires this tall and wide.
 
#15
So it seems that from mfg. to mfg tire sizes are not as they are being represented as.
Which brings the question of, ,,why not?
Wouldn't this be considered as a fraud?
 
#17
I thought I had my tires pretty well setup but now I see I may have misjudged a few things. Oh well, I'll run my 315x70-17's until they wear out. They do get a lot of wear running in the desert and on the rocks we have to deal with. They are supposed to be 34. 1/4" in size and, yes they do work. I'm not sure my gears would work as well if I had larger tires. Interesting post, regardless. Tells me I'm still in elementary school when it comes to off-roading.
 
#18
You want to measure the loaded radius of the tire at normal street pressure, not the overall loaded diameter. Typically you can subtract 1/4-inch from the loaded diameter to get a reasonably accurate radius if you run relatively high pressure (>30psi) on the street. If you like to run a lower pressure, subtract 1/2-inch from the loaded diameter instead.

Or you can use a GPS to measure your actual highway speed, and then make corrections to your speedo by trial and error until you've gotten it dialed in.

I found that my 315/70R17 Cooper STT Maxx tires to be ~1/2-inch taller than my 315/70R17 BFG A/T-KO's were.

My next set of tires will likely be Milestar Patagonias. I'm still debating between 37x12.5R17 and 38x13.5R17 because I'm not sure how true to size they run, or how they compare to more well-known tires. I'm also trying to find a set of wheels I like that also have little enough backspacing (4.5" or less) for tires this tall and wide.
I’m interested in the milestars too in 38s. What I’m also curious about is the tire size to gear ratio charts. When they state the tire size are they going by linear or what’s stamped on the side of the tire. If it’s linear inches then the 4.88 would work great for the 38s since they’re pretty much 37s
 
#20
It’s a well known fact that the BFG tires run small but how small are they really.

Here are a few pics of a 315/70-17 Cooper STTPro on a stock Rubicon Wheel (17x7.5}next to a 315/70-17 BFG KO2 on an AEV Wheel (17x8.5) and. 37x12.5-17 BFG KO2 on Raceline wheels (17x9). Viewers should note that the 315 BFGs have 50k miles on them at 33psi and are mounted under weight of a Light Build JKU. The 37” BFGs have 20k miles on them at 28psi and are on a heavy build JK. The STTPro is brand new not installed on a Jeep at 30Psi.

The 2 315s


315 and 37


What are y’alls experiences with tires that run big or small?
I run 37 Mickey Thompson and they measure at 36.5
 
Top