Originally Posted By: Quattro Pete
Originally Posted By: Ken2
Taking some numbers out of the air---say the tire carries 1000# and its footprint is 4 square inches. That is a pressure of 250 pounds per square inch on the base for traction. Increase the footprint to, say, 6 sq. in., and you have reduced the pressure on the base to about 167 psi. Less grip.
Based on what I've read before, the actual area of the contact patch does not change very much, but it does change. What changes the most is the shape of it.
Maybe Capri can comment on that...
First, let me state that the difference caused by tire size is small compared to the difference between makes and models of tires.
Put a different way: Choosing the right tire for your conditions pays much, much more dividends than changing tire size does.
The size and shape of the contact patch varies quite a bit. No, you can NOT calculate the size of the contact patch - not even an estimate. It can ONLY be determined by actual measurements.
So if you have the situation described here (2 different sized tires carrying the same load), the size of the contact patch is ABOUT the same, the actual size (and shape) is governed by the make and model of tire (or more accurately, by the engineer who designed the tire). It would be a bad assumption that the size is the same, but in the big scheme of things, that assumption is sort of true. The problem is that many people think that size of the contact patch is important for handling and grip - and compared to tread compound, it is not.
Oh and you've got to be very careful about the difference between a P245/75R16 and an LT245/75R16. Completely different animals - and the Nissan Pathfinder came with P type tires, not LT's.