Ok this is probably a little AR but I noticed this. Here is the setup:
2008 2LT with Auto headlights and foglights.
When the HHR is in the daylight, with the Auto headlights on, when I turn on the fog lights the "MPH" light on the speedo dims. No other interior light does this just the MPH light. Kinda weird and wondering if this happens to anyone else's HHR.

 

Can't say that I've noticed this in the situation you mention. Are you also aware if in the auto mode, when the headlights come on, the related dash lights dim?
I'm sure in the daylight you just aren't aware of this, harder to perceive.

 

Yes but in full daylight they come back up brighter. I specifically looked and only the MPH light dims. It is just wierd.

 

I checked mine today, what you describe does not happen on mine.
The MPH light doesn't dim when turning on the fog lights.....weird.

 

mine does it, but i think they all dim, just the MPH is more obvious because it's bigger, and white, while the rest of my gauges are red and such (SS gauges look different), so the lighting on them is nearly impossible to see during the day. you should also notice the little LED light on the roof comes on when you turn on the fogs during the day

 

Turning on the fogs is the same as turning on the parking lights..otherwise day lites are just that, no other lites..so with fogs on whatever setting you have the dimmer at is what you will see at the guages...

 

Don't know about the "newer" models, but this statement is not true for the 06 model.

The only time that the fogs affect the interior lights is when the light sensor on the dash thinks it is "night". If I turn the fogs on during the day, the interior lights are not affected.