If your weight is 235 pounds and you have 70 psi in your tire, the tire will have to flatten out so there is a contact area on the road of 235/70 square inches. That's about 3 square inches. In other words if you have fairly narrow tires the contact will be 6 inches long by 0.5 inch wide. Yep - that tire's gonna look quite flat !!

For your weight you might consider a tire that can go to 120 psi.