i am running out of hot water during shower. replaced dip stick. same problem. replaced tank. same problem. troubleshooting by plumber thought problem with gas pressure. per gas company pressure good. only getting 6 minutes of hot water from a 40 gal tank when running water from spigot in tub. no apparent leaks in pipes. help?

Hmm, that's interesting. If the plumber on site thought it was a gas pressure issue, are you maybe trying to run natural gas through a propane orifice?? That is a fairly easy fix, if so.

When you run out of hot water in the tub, is the water hot in other parts of the house? If so, then it may be the tub faucet itself.

Or maybe you just have a helluva shower head??

The dip tube and sediment in the tank are probably the most common causes and it sounds like you've already addressed those.

Sounds like the hot and cold pipe is reversed. Your only drawing off the bottom of the tank.
That would be the only other reason.
If the gas is working, and the setting is ok, then that would be the answer.
did you check the temp of the water on the tank. Perhaps the gas valve thermostat is not working properly. It may be shutting off to early and the water temp is not hot enough to deliver what you need.

One other reason. Teenagers. Take your shower first!