Ok, not really flat. I've got a Specialized Crossroads Sport with 700x45 tires. The sidewall says 55-80 psi. I pumped the tires up to about 75psi based on the gauge of a floor pump. My gauge is from a HD shop and reads up to 60psi. When I use the gauge it goes past the 60 and pegs so I believe I have 65-70 psi or so in the tire. By feel, it's hard as a rock. Anyway, when I am on the bike and I look down and back at the tire it looks almost flat. I am about 235 pounds so it's carrying a lot of weight. I know I'm not riding on a flat tire but wonder if I need to get it up to 80psi or if it's ok as is.

I'd fill it to max, and check it with a known, good gauge.

Floor pump gauges are notoriously inaccurate.
Based on your description, I'd say the Harley gauge is probably right. Throw some more air into 'em.

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.

Exact - if he had a unicycle with a narrow tire on it.

He's probably got 60% or so on the back wheel, so it's (235 + bike weight) * 60% = 155 lbs(ish).

All that being said, if the tire looks flat, get a good gauge and fill it up more.

My tires are 700x45 so not skinny by any means but not super wide like some I've seen. My gauge is a supposedly good gauge from the HD dealer. The highest marking is 60 and the tire reads past the 60 and pegged so I presume that's 65-70 psi. At some point I'll have to get a gauge that reads up to 90 so I can measure on the dial up to the max rating of the tires.

It may be a good guage for other applications, but since its range does not cover your operating pressure it is not a good guage for your bike.

Also, pressure guages can sometimes be damaged by using them above their range - more commonly with the dial type of pressure guage than the stick type.

HD? you mean one sold for bigger volume, low pressure, Motorcycle tires?

Mieser , a dial gage, is my reference gage.. 1 p/v, 1 s/v.

on a Bike Tour, I brought a plastic Zefal one. [it's p/v]

Just ordered one each of Mieser dial type gauges so in a few days we'll know what we know.

I seem to be in the minority here, but with 45mm tires I don't think you have to go anywhere near 100psi, or even 80. I weigh 245-250 and run 35mm Paselas at 80-85, 50mm Schwalbe Big Bens (actually 45mm wide on my wheels) as low as 45psi. I ride dirt and gravel roads a lot, the roads here are marginal, and I haven't pinch flatted in 15 years.
There doesn't seem to be a measureable difference in speed in the real world--I've done the same 25-mile RT commute more than 2000 times in 30 years, on a bunch of different bikes with tires from 21mm at 120psi to the 50mms at 50, and my fastest time was on an old Bridgestone mountain bike with 1.9-inch Ritchey Quads, a combination tire from so long ago that nobody else even remembers it.