Stereo & Electronics - Voltage Gauge Issue

My voltage gauge reads around 12 volts when car is hot with this summer heat but when I put a meter on the battery it says 13.4v....why is my dash voltage gauge almost in the red :confused:

I also have the same problem. I hope some one chimes in.

I'm adding an external regulator to see if that helps with the heat issue

i had a similar issue, and i installed and use a "scangauge" http://www.scangauge.com/ this shows that im actualy running 14.0 vlots driving, and 12.7 when just acc is on for stereo or etc. 150 bucks, but it has ENDLESS digital gauges like intake air temp, volts, tps, lod, hrp(est) mpg and it reads and clears trouble codes. as a side benefit, u can change display color on a whim, and it diplays 4 gauges at once, i will try to upload a pic im new here....
long story short, my stock gauge just wasnt accurate.