❓ Tire inflation question

[spanishmack]

So, mainly just a sanity check here. I have Toyo Open Country R/TS in 37x13.5R17 size. According to the manufacturer's site and a few other sources, actual size is 36.5" I emailed Toyo about what proper inflation was for my Sport S/w Max Tow pkg was and they came back with 26 PSI for those tires. That's what I've been running for a while now. I thought I'd check the computer just to see what the dealer had set for tire size and it was set to 37. I changed it to 36.5. Speedo seems good still. GPS speed seems to be averaging about 1 mph less than speedo, but I had never really checked it before so I have no basis there.

I decided to actually measure my tires and they measure out at about 35.5." My question is, does that seem about right for those tires, given only 26 PSI inflation? I think that seems to jive, but still a litte off-putting at a sub-conscious level to have 37" tires measuring out at 35.5."

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[GuzziMoto]

It is common for tires to measure out smaller than the advertised size, and to measure out even smaller under the weight of the vehicle. But I am surprised that a Toyo 37" tire would measure out to 35.5". That is what I expect from BFG KO2's, as they are notorious for running small. If you need something to make you feel better, measure some 35" tires, they can run as small as 33.5" in actuality.
By "GPS", do you mean the factory GPS or aftermarket. The factory GPS likely shows the vehicles speed signal, not the actual speed. To see actual speed with GPS it would have to be a standalone or non-factory GPS.
There is a chalk test you can use to check inflation pressure. By marking across the tread with chalk and watching it wear off you can see if the tread is evenly contacting the road surface. Or you can do the math, calculating the optimum pressure by dividing the max load by the max pressure and then multiplying that by actual load. A third option is to watch air pressures, they should increase by about 10 to 15 percent in use. Not enough increase means the pressure is too high while too much increase in pressure means the pressure is too low.

 

[spanishmack]

It is common for tires to measure out smaller than the advertised size, and to measure out even smaller under the weight of the vehicle. But I am surprised that a Toyo 37" tire would measure out to 35.5". That is what I expect from BFG KO2's, as they are notorious for running small. If you need something to make you feel better, measure some 35" tires, they can run as small as 33.5" in actuality.
By "GPS", do you mean the factory GPS or aftermarket. The factory GPS likely shows the vehicles speed signal, not the actual speed. To see actual speed with GPS it would have to be a standalone or non-factory GPS.
There is a chalk test you can use to check inflation pressure. By marking across the tread with chalk and watching it wear off you can see if the tread is evenly contacting the road surface. Or you can do the math, calculating the optimum pressure by dividing the max load by the max pressure and then multiplying that by actual load. A third option is to watch air pressures, they should increase by about 10 to 15 percent in use. Not enough increase means the pressure is too high while too much increase in pressure means the pressure is too low.

GPS speed is from Android auto, so that should be from my phone. I'm not really too hung up about this. Tires look the same as they've always looked, just odd sometimes when you get real numbers for things.

 

[D_JT]

Tire PSI depends on a lot. Vehicle weight, tire size and what load rating the tires are. Also, tires will always measure "smaller" when mounted on the vehicle and on the ground, obviously the side wall bulges out resulting in a "shorter" than advertised tire. But I have the same tires on my diesel JT. They are 315/70/17 (35x12.5) in load D. I run 30-32. The best way to know is to do a chalk test.

 

[Badunit]

Get on an interstate and compare your odometer/trip meter vs the mile markers. The further you go, the more accurate this will be. Divide the actual miles by trip miles. Multiply that by whatever is currently in the computer as tire size. That's your actual tire size. Plug that into the computer.

Inflation doesn't affect the tire size in regards to the speedometer/odometer. It is the circumference that matters and inflation doesn't change that. One revolution = you traveled the entire circumference of the tire.

 

[MPMB]

Tire Mfgs. need to clarify their marketing. 35s, 37s, 40s, etc. should be referred to as a "class" rather than a dimensional tire.

What's the max PSI of the tire? Did you measure the tire size at that PSI?

If we all got pissy enough, we could hire a lawyer(s) and sue the industry, just like they did when the idiots found out a 2x4 isn't 2" x 4". Lawsuits make everything better. /s

The production method of tires makes it somewhat difficult to 100% guarantee all tires will be the same size after mounting & heat cycles. There's going to be fluctuations. Radial tires, which street tires are, are easier to get smaller variations because of the construction of the tire when compared to the older bias-ply tires.

A wide rim will stretch the sidewalls out, making the tire shorter. A narrower rim makes the tire taller. So there's another aspect that can "change" the size.

In racing, the tire sizes we had were stated to be 26.5" on the left, and 27" on the right. However, for circumference measurements, the lefts could range anywhere from 81.5" to 84.75" and the rights could be from 85" to 87.75". Using maths, a 26.5" tire should have a circumference of 83.25" and a 27" tire should be 84.8.

When I went to 37s, I changed the JT's setting to 36.5 because the tires weren't true 37s. And then I drove it and compared the speedo to a GPS speedo on my phone. Off by 1-2mph. Changed the tire size again, down to (I think) 36.25" or 36". Speed matches with GPS. And it matches with the map apps between me & my wife's phones.

Do you know where speed doesn't match? At those electronic speed signs. I'm not talking about the trailers with the radar checks, but the permanent signs along highways. Those are always slightly higher. Nice try, Big Gov't.

 

[AustyPosty]

It is common for tires to measure out smaller than the advertised size, and to measure out even smaller under the weight of the vehicle. But I am surprised that a Toyo 37" tire would measure out to 35.5". That is what I expect from BFG KO2's, as they are notorious for running small. If you need something to make you feel better, measure some 35" tires, they can run as small as 33.5" in actuality.
By "GPS", do you mean the factory GPS or aftermarket. The factory GPS likely shows the vehicles speed signal, not the actual speed. To see actual speed with GPS it would have to be a standalone or non-factory GPS.
There is a chalk test you can use to check inflation pressure. By marking across the tread with chalk and watching it wear off you can see if the tread is evenly contacting the road surface. Or you can do the math, calculating the optimum pressure by dividing the max load by the max pressure and then multiplying that by actual load. A third option is to watch air pressures, they should increase by about 10 to 15 percent in use. Not enough increase means the pressure is too high while too much increase in pressure means the pressure is too low.

The whole KO2's "running small" is a common misconception, its literally all tire brands that "run small". I think this misconception came from when Ko2s were new and literally everyone was running them, and just assumed it was the just the Ko2 that ran small, due to a lack of experience with other off-road tires.

Evidenced by the Toyo (Aka a brand everyone claims is true to size) being 35.5 in their 37in size. My 37" Ko2 is also 35.5. Not a dig at Toyo either, this is just every tire brand and its normal.

 

[antihero23]

My 37 x 12.5 KO2s measure right at 36 on the money. I run em around 32 psi for driving in a big city. I find for my ride, 30-32 psi is a good spot. Smooth and not too bouncy.

 

[GuzziMoto]

The whole KO2's "running small" is a common misconception, its literally all tire brands that "run small". I think this misconception came from when Ko2s were new and literally everyone was running them, and just assumed it was the just the Ko2 that ran small, due to a lack of experience with other off-road tires.

Evidenced by the Toyo (Aka a brand everyone claims is true to size) being 35.5 in their 37in size. My 37" Ko2 is also 35.5. Not a dig at Toyo either, this is just every tire brand and its normal.

No. I have run the same size tire but different brands. My KO2's ran smaller than the other brands. And I am not alone in this experience.
For example, my 37" KO2's measured out between 35.25" and 35.5" installed. While my 37" Nitto Ridge Grapplers measured out at 36.5" installed. This difference is reflected in the different tire size settings required to get the correct speedo reading.
I can't speak to the other guy saying his Toyo 37's were 35.5" That is surprising. But other 37's I have measured were larger than my KO2's.
Tires do run smaller than advertised for the most part. But the KO2 seems to run smaller than most. That is just what it is, and I do like the KO2. From what I hear the KO3 is not quite as bad.

 

[AustyPosty]

No. I have run the same size tire but different brands. My KO2's ran smaller than the other brands. And I am not alone in this experience.
For example, my 37" KO2's measured out between 35.25" and 35.5" installed. While my 37" Nitto Ridge Grapplers measured out at 36.5" installed. This difference is reflected in the different tire size settings required to get the correct speedo reading.
I can't speak to the other guy saying his Toyo 37's were 35.5" That is surprising. But other 37's I have measured were larger than my KO2's.
Tires do run smaller than advertised for the most part. But the KO2 seems to run smaller than most. That is just what it is, and I do like the KO2. From what I hear the KO3 is not quite as bad.

Sorry but I disagree. If you compare a 37" Nitto Recon Grappler AT (Not RT like the Ridge Grappler), it also runs around 35.5-35.7. MTs and RT tires run larger than all AT tires, thats not new info.

 

[TARHEELKIDD]

So, mainly just a sanity check here. I have Toyo Open Country R/TS in 37x13.5R17 size. According to the manufacturer's site and a few other sources, actual size is 36.5" I emailed Toyo about what proper inflation was for my Sport S/w Max Tow pkg was and they came back with 26 PSI for those tires. That's what I've been running for a while now. I thought I'd check the computer just to see what the dealer had set for tire size and it was set to 37. I changed it to 36.5. Speedo seems good still. GPS speed seems to be averaging about 1 mph less than speedo, but I had never really checked it before so I have no basis there.

I decided to actually measure my tires and they measure out at about 35.5." My question is, does that seem about right for those tires, given only 26 PSI inflation? I think that seems to jive, but still a litte off-putting at a sub-conscious level to have 37" tires measuring out at 35.5."

I just purchased the Toyo Open Country AT III in the exact same size. It is an E rated tire whic factors in on the PSI. The dealership put in 40 PSI? I am not sure where they got that number from as my door label calls for 36, SMH.. I too went directly to Toyo and they advised me to use 26 PSI as well for my Jeep, unloaded at the factory weight. I have a full-sized spare in the bed of the truck and a steel bumper and 12,500 LB winch with a steel line and settled in at 27 PSI. So far, it seems perfect. Time will tell. I replaced the same sized Nitto Ridge Grapplers. Per Nitto's instructions, I kept them @ 28 PSI. They rode great, wore evenly and I actually got close to 60,000 miles out of them with a safe amount of tread left. I only replaced them because they were 6 years old and showing signs of dry rot.

 

[S JEEPN]

The manufacturers often have a chart that has pressure versus actual load to put you in the ballpark. i have used the chalk line drive down the street and back test the only issue may be doing it with mud tires where you do not want the outer lugs thumping on the road or if you change the load in your vehicle a lot you would have to keep that in mind. They also have rim width recommendations and actual tire dimensions but as mentioned once you match a tire with a wider or narrower rim you change things. I remember seeing an article in a magazine that tested tires at various pressures to see if the distance traveled at low air pressure differed from the distance at street pressure. They came to the conclusion with the lower inflation pressure the tire did make more rotations to go the same measured distance calling tires variable gearing devices never tried it myself.

 

[guntrust]

My 37 x 12.5 KO2s measure right at 36 on the money. I run em around 32 psi for driving in a big city. I find for my ride, 30-32 psi is a good spot. Smooth and not too bouncy.

I run about the same with my 37 x 12.5 K02's, maybe a little more...between 32 and 33.

For some reason the Jeep dealer i go to keeps overinflating even though they know these are aftermarket. They've inflated as high as 38 and recently about 35 (cold).

I like the idea of the chalk test several mentioned. I've just been eyeballing it for full tread contact with road.

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