The tires I chose for our experiment were Mickey Thompson Baja Boss all-terrain. The wide tire is the familiar 315/70R17, and the narrow tire is 255/85R17. Some people may point out that these two tires are not equal in diameter. The 315 calculates to be 34.4 inches, and the 255 is only 34.1 inches. Yes, the math is correct, but if you read the spec sheet, the 315’s actual diameter is 34.5 inches, while the 255 is 34.6 inches. I also measured both tires carefully using squares and can confirm those numbers. So, the diameter of the sidewall heights is pretty much identical. These tires also share the same Load Range E, which means the tire carcass has the same stiffness. This is especially important because we will compare how they behave when aired down. Therefore, the only difference between these two tires is the width, making this an actual controlled variable experiment.
The 315 and 255 millimeters refer to the overall section width, sidewall to sidewall, but what we care more about is the tread width—the stuff contacting the ground. For the wide tire, I measured 10.2 inches, and for the narrow tire, I got 8.5 inches. To calculate the contact patch, I applied block printing ink to the tread and painted it on paper. Starting with the wide tire at 40 PSI, intuitively, I was expecting a short rectangle that looked something like this, but instead, this is what I actually got. It was almost a perfect circle. Dropping down to 35 PSI, which was the recommended pressure for my setup, the circle got more prominent, but still, we didn’t have the entire tread width in contact. I continued to reduce pressure, and the contact patch got both longer and broader. Only when I aired down to 5 PSI did we finally have the entire tread width in contact. Here is what 0 PSI looks like. Many experienced wheelers will tell you that airing down does not increase the contact patch width; it only grows longer like a tank track. That was also what I believed all these years, but my data showed otherwise, at least for this particular tire.
I then swapped in the pizza cutter and repeated the exact measurements. Starting at 40 PSI, the narrow tire also had a round contact patch, and just as expected, it was narrower but at the same time longer. As we aired down, the contact patch also grew longer and broader, just like the other tire. If we plot the contact patch length versus tire pressure, we can see both tires behave very similarly, except the narrow tire was consistently longer. For contact patch width, it was the other way around. Now, I’m sure you are eager to know which one has the more significant area. Because the contact patches are not rectangular, we cannot simply do width times length. So, I scanned all my block printing in proper scale, manually traced the border of each contact patch, and then used my vector editing software to compute the exact area in square inches. Here is the area versus pressure curve for the wide tire, and here is the curve for the narrow tire. They were almost on top of each other with no practical difference. Many people naturally assume a wider tire means more rubber on the road, but as we just saw, narrow and wide tires yield the same contact patch area. We are just trading between width and length.
Now, what about a completely different tire? Does the brand and model affect the contact patch? This got really interesting when I measured my BFGoodrich KM3 mud-terrain tire, which is identical in size and Load Range E as the wide Baja Boss. I found its contact patch has a totally different shape. It was more rectangular—wider and shorter—but to my surprise, when I traced out the area, the KM3 was actually 10 to 20 percent larger, especially at very low pressure.
I conducted two different experiments on this. In the first experiment, I lowered the tire onto a piece of rock and measured how much the tire deformed at various PSI. I made sure the rock was centered so the tire bulged out equally on each side. I measured the gap between the tire and the ground by stacking wood shims with various thicknesses. The rock itself was 5.06 inches tall, so how much the tire caved in was 5.06 minus the height of my filler gauges. I jacked up the tire every time I changed the pressure because I found the deformation of the tire would affect the pressure reading, especially at very low pressure with large deformation. Yeah, it was much work, but this gave me the most consistent and accurate measurement. Starting with the wide tire, the change was very minimal at higher pressure. Dropping from 40 PSI all the way down to 15, the tire deformation barely even changed. Only at 10 PSI and lower did we start to observe some meaningful tire flex. But at pressure this low, the risk of spinning is significantly higher. For 90 percent of us, 15 PSI is where we operate, and at this pressure, we don’t have any flex.
Now, let’s see how the narrow tire compares. Right off the bat, at 40 PSI, we already observe more tire flex, and it gets better. At 20 PSI, which is still fairly conservative for off-road, we already have doubled the tire flex as the wide tire. To achieve this much tire flex, the wide tire has to drop down to around 7 PSI, and that is definitely beadlock territory. Interestingly, near 0 PSI, the wide tire actually caught up. My theory is that at extremely low pressure, the tire deformation across the width started to kick in. Basically, it became more of a 3D deformation. The 255 tire wasn’t much more expansive than this rock, so its 3D effect wasn’t as pronounced. But what if the rock is broader? A rock ledge or staircase type of obstacle is very common. These obstacles can easily span across the entire tread. How would the wide versus narrow compare in that situation?
Now, the obstacle spans across the entire tread. Both tires deform less. For the wide tire, I measured identical tire flex from 40 PSI all the way down to 15. The tire was visually deforming, but the bottom of the tire was just approaching a flat line. It didn’t mold around the tube. Even at 15 PSI, the practical tire flex was still merely half an inch. Only below 10 PSI did we start to see more increase, but like I said before, 90 percent of us would never get this low. And now, it’s time for the pizza cutter. Pizza time! Right off the bat, the narrow tire has the same amount of flex at 40 PSI as the wide tire at 10 PSI. As we air down, the pizza cutter also started to deform much sooner. At 15 PSI, we already reached 1 inch of deformation, whereas the wide tire has to drop down to 5 PSI to achieve the same. When we dropped the pizza cutter to 5 PSI, it completely molded around the tube and touched the ground. This was something the wide tire never achieved, even at zero PSI.
At this point, I was fully convinced the pizza cutter was the performance choice, at least for what I like to do. But man, I still couldn’t get over the look. So, it’d be so nice if there’s a tire that can both flex over rocks and look thick at the same time. But what if I told you there is such a tire, and I already have it? Remember the KM3 having a very different contact patch? Guess what? It also behaved very differently over obstacles. For the rock experiment, despite the KM3 being 315 wide, it flexed even better than the 255 Baja Boss. My theory is that this is thanks to KM3’s Linear Flex Zone design feature. Few tires out there actually advertise to have a similar feature, so BFG clearly knows how to rock crawl.
For the second experiment with the tube, the KM3 started out flexing the least at street pressure, but it quickly caught up as we aired down. Although it didn’t surpass the pizza cutter, the KM3 was still significantly better than the Baja Boss with the same size. So, my KM3 was indeed the best of both worlds. More importantly, they were already on actual beadlock wheels, so I could actually utilize the ultra-low pressure.
Now, which Baja Boss AT did I end up getting a complete set for my RV? My heart still goes to the wide tire, but as a mechanical engineer, I followed what the data suggested. I chose the pizza cutters. A 35-inch pizza cutter is a scarce size in short supply. Besides the experiment we did, each 255 Baja Boss was seven pounds lighter than the 315, and all seven pounds came from the tread, which has the most significant impact on the moment of inertia. The narrow tire is also much easier to fit, especially on an IFS Toyota. If more of us consumers realize the benefits of pizza cutters, we will see more tire manufacturers start offering them again.
