Trailer tire -

[nofearengineer]

The Marathons are good to 65 mph, and they do indeed say to put an extra 10 psi in them if you want to run up to 75.

The reason for more air is:

The tire sidewall bulges out slightly at the bottom when your boat and trailer are sitting on it. Duh.

As the tire rolls, the the sidewall is flexed over and over again, as each spot on it passes through the bulge area. This is what generates heat. It's like bending a piece of stiff wire repeatedly until it gets very hot. That's why I always get out and check the temps with my hand. Regardless of what any piece of paper tells you, if the sidewalls are hot, you're doing something wrong.

Adding the extra air reduces the bulge somewhat, reducing the heating.

The Marathons are supposed to inflated to 35 psi hot (up to 45), but I have heard of guys running them at 65 psi. Just begging for a catastrophic blowout.