Jeep Gladiator BFGoodrich KM3 39" Tire - Chalk Test
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- เผยแพร่เมื่อ 17 พ.ค. 2020
- In this quick video, I show you how I finally landed on the ideal tire pressure for my giant 39x13.50x17 KM3's. Unlike the original recommendation of "just run what's on the door frame sticker," the actual way to dial in the ideal tire pressure on aftermarket much larger tires isn't just a cookie-cut approach from OEM spec. The Chalk Test is the preferred method to assure the thread is laying down flat on the pavement creating even wear.
I landed on 30PSI Front and 28PSI Rear. These are COLD PSI temps, and with this much tire, the typical warm-up PSI ranged around 5 actually.. Running around town before I did this chalk test (always want to do the chalk test warm), they were all around 35 PSI. I deflated the rears to 33 PSI to fix the slight over-inflation results, which translated down to cold PSI, will be around 28.
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Because the front is heavier
I run 26-28 cold on my 39” KM3 2 door JK runs about 32-33 hot. I could probably even go down 2 more psi. But they wear and drive great.
Yea, I have settled in (since this video) around 30 PSI cold front and 28 PSI cold rear for them to wear evenly once warmed up.. Obviously is going to be different for every vehicle since you have weight to consider, etc.. I do love the tires though!
Great video...how tall are the tires mounted on the JT? I saw a recent video where a guy measured a 37" km3 mounted and it was 35 1/8" tall. Is the 39" closer to 37" mounted?
Thanks! Yes, it's a little larger than 37", but you're right, definitely not 39" mounted with 28-30psi.
Did you have to upgrade your axles? And if you regeared to what did you to? Thanks in advance
I added a truss and C-gussets up front, but still running stock JT axles. I specially went with the BFG KM3 39”s due to weight as they are 88 pounds per tire vs 104 for the Nitto 40” (wasn’t worth the extra stress and load for 1”). Nitto 37”s I believe are still 83-84 pounds per, so 39” at 88 is quite good.
Gearing I went with 4.88”s. They were perfect until I added a ton of payload weight (Decked system, rack, suspended full size spare and RTT). As long as you aren’t adding overloading gear or towing a lot, 4.88”s are great. Otherwise, I’d recommend 5.13”s
do the bfg’s run true to size? what was the size after mounting?
Nah, definitely not. They are true 37” when mounted, under weight, and 30PSI front and 28PSI rear (cold).
What wheels are those? Nice rig btw
Fuel Coverts! Thank you :)
Wheel width? A common oversight when chalking a tire. Narrower the wheel, the lower the pressure to avoid crowning of the tread.
I'm curious to learn more about your theory, because the whole point of doing the chalk test is to see where the tread is making contact with the surface of the road, ALL things considered (wheel width, current pressures, vehicle weight, tread design, tire width, etc..). How would the width of the wheel change the chalk test results?
@@Chromedonkey it isn't MY theory, it is an engineering fact.
On a 13.5" wide tire, the closer the wheel is to 13.5" , the more pressure you can run and still use the entire tread width. The narrower the wheel, you'll actively have to run low pressure to use the entire tread width.
Other factors like vehicles total weight, and tires load rating also make a difference to figure your correct PSI, running a delicate balance if you really care.
The downside to narrow wheels, wide tires and low pressure to use the entire treads width; radials lay out like a track - so you also significantly reduce fuel economy and change handling characteristics by way of a massive contact patch. The benefit to narrow wheels and a wide tire - it acts like a poor mans beadlock.
FWIW - I'm on Stock GladRubi wheels and 37x12.5 D rated KO2s. To use the entire treads width, I'd need to run 19 psi daily - yes I've chalked them. At 19 psi, they're unmanageable on the road, especially in parking lots - by way of a huge contact patch. I run a comfortable 30psi, reap the benefits of near stock fuel economy on highway, and superb bead retention below 10 psi offroad!
The long version of my original question - whats your wheel width?
Also depends on car's weight
Exactly! That's the entire point of this test, you can't just go by what's on any door sticker.
Isn’t it the other way around? If the edges aren’t touching its under inflated?
No, you have it backwards.. If the tire is OVER inflated, that will cause the edges to not come in contact with the pavement. If it's under-inflated, more of the contact patch hits the surface. This is why you air down while on the trail instead of airing up.
@@Chromedonkey ha okay awesome thank you
@@dad3562 Anytime! I, admittedly, watched several videos on how to do this properly before I did it myself. I have NO clue if by doing this my thread wear is what it is, or if it's just the tire; however, I must say, these KM3's are 200 miles away from having 30,000 miles on them, and I would guestimate at least 1/3 if not 1/2 of the tread is still rocking!! VERY impressed w/ the tread life of this tire vs. my previous experiences with BFG KO's.. Night & day!
@@Chromedonkey nice that’s awesome! Me too on KM3’s and I think I under inflated my rear tires since it was only wearing on the outer edges. I was running at 26 PSI on a new Ford Ranger. Front was kind of wearing so I went to 30 and it seemed to improve it but have since went to 35 since it looked like the middle tread looked untouched. I’ll have to do the chalk test again for sure, it’s fun lol.
What gearing are you running?
4.88’s
@@Chromedonkey Thanks. Assuming your harpy? I live in Atlanta area and curious if i would need to go to 5.13. But if 4.88 is good around here then I'm good too. Lol.
@@JEEP-N-DIRT think it depends on if you do a lot of towing.. If I towed all the time, I’d likely be on 5.13’s. For me, 4.88’s are perfect :) (and a little stronger actually)
@@Chromedonkey Thanks. I get 15mpg avg around here with 4.88 and 37's (actual 35.8inches). What's your mileage?
@@JEEP-N-DIRT 13 for the most part :)
Great video but you got to lower down that stupid music that way we can hear you better nice Jeep
No doubt! Lessons learned since doing some of these earlier vids... Thanks for stopping by, and I appreciate the compliment on the rig.
@@Chromedonkey I'm curious cuz I'm new to this whole Jeep world I've got a 2021 gladiator I just put a set of 33x12 and 1/2 by 17 they are f rated nexen rodian MTX tires they drive real nice but these things were rated maximum 80 PSI now the chalk test is a great idea so I got them at 40 PSI right now but I got to admit the fuel mileage went down what's your recommendation should I put them up to 50 and get better fuel mileage be in the way gas cost so much today truly appreciate your advice thank you
Music wasn't really that bad just was trying to hear the good advice That's all 😀
@@carloscarmona7667 I don't make any false representations to be a tire specialist. That said, any tire will give you better mileage if you add more air to it.. This is simply due to the fact that the more air you pump in reduces the amount of rubber that ever touches the ground under normal driving. This is why the chalk test, to me, is the defacto standard. It takes into considering your vehicle's weight, how it is distributed, tires themselves, etc.. The idea is, when you are just sitting there under normal conditions, you ideally want an even patch to touch the surface of the road. This is where the chalk test gives you the insight needed to make tweaks.. Could always increase them back to regain some MPG's, though you might be sacrificing slightly uneven tread wear (which might be a justifiable consideration based on how many MPG's you pickup) :).
*Also, if you scroll through the comments, @TankTheGladiator made an additional consideration regarding wheel width.
@@Chromedonkey okay I will check it out thank you so much for all your help truly appreciate it 👍