Driving through downtown Atlanta is, at best, questionable on ANY weekday from about 3PM until about about 7PM. On Fridays, that timeframe will expand.
We are in Gwinnett (northeast side of town, out I85). Whenever we want to drive through downtown, we always check this website.
Georgia Navigator
It will warn you of any incidents/accidents that are affecting traffic. You can also click on NEWS to find out about any planned construction.
If you have access to the internet on your cellphone, then you can use this website as you are approaching Atlanta to help decide whether to drive directly through downtown Atlanta or whether to drive around on the perimeter (I285).
If you plan to drive through down town Atlanta on I75/I85 ANY day after 3PM, it will, at best, be congested traffic-wise. You should be comfortable driving in very heavy traffic. If you have any concerns about doing that, then you should take I285 around the city. There are crazy drivers on both routes, but the downtown route is a bit more nerve-wracking (in my opinion) primarily because the lanes are a bit narrower through downtown.
Basically -- you can NOT look at the mileage through downtown Atlanta, and estimate your driving time accordingly. If you are driving through Atlanta, just be alert, try not to get too tense, and take any traffic delays in stride. Don't have any preconceived notions concerning how long it will take to get through the city. I would suggest that you tune in to AM750 for traffic reports.
If you can get from I75 or I85 on the top end of the perimeter (I285) to
I75/I85 on the bottom end of the perimeter within about 30 minutes driving time, then you've done pretty good.