Salzburg- Grossglockner round trip.

The driving time (without stops) would be at least 5:30 hours, assuming you go from Salzburg (via Kaprun) to the Grossglockner alpine road, heading south over the pass, go to Spittal/Drau, and from there straight back to Salzburg via the A10 highway.

That is the route:
https://www.google.com/maps/dir/Salzburg,+%C3%96sterreich/Heiligenblut,+%C3%96sterreich/Spittal+an+der+Drau,+%C3%96sterreich/Salzburg,+%C3%96sterreich/@47.3160181,13.2156172,9z/data=!4m14!4m13!1m2!1m1!1s0x47769adda908d4b1:0xc1e183a1412af73d!1m2!1m1!1s0x47776acfa2ce7fd7:0x8d3186646f07359e!1m2!1m1!1s0x4770a2e1fd11a72d:0x8de1512ccc09a8ee!1m2!1m1!1s0x47769adda908d4b1:0xc1e183a1412af73d!3e0

There is a big multi-story garage at the top of the cable car (the cable car is unusual there in that it takes people from a high point down to the old (1950s) glacier base, which is now several hundred meters removed from the glacier itself due to glacier retreat/rising temperatures. There are restaurants and other facilities up there. Make sure to stop at the different vantage points along the route.

You don't need to backtrack, there is a faster route with a tunnel taking you through.

Hello, Small world. I was plotting a similar road trip yesterday. In several weeks we plan to approach the GrossglocknerStrasse from the NW, coming down from Chiemsee -- so the routing will be a bit different. Like you, we will then head back north, by Salzburg, to Mondsee. Disclaimer: this is all based on websites and maps; I expect more reliable advice will follow from the Austria Forum regulars.

I would suggest using GoogleMap or ViaMichelin to investigate something like the following:
1. Route from Salzburg to Zell Am See, then the short drive down to the northern end of the GS at Ferleiten. Rts 21 to 78 to 311 looks promising. Others with real experience will hopefully give you better suggestions.
2. Travel the GS to southern end at Heiligenblut.
3. (The following is our scheme) from there: Rts 107 and 106 east to Seeboden; get on A10 for a relatively straight shot back to Salzburg.

You should have plenty of time with a moderately early start. Looks like the drive to the start will be about 2 hours. Others on the Forum have suggested 2 hours+ for the GS drive, but dining and admiring the views probably make this highly variable. The return to Salzburg will be 2.5 hrs or so, depending on route. (One is a partial backtrack on the GS.) More info on the Grossglocknerstrasser website: https://www.grossglockner.at/gg/en/index

Wow... Thanks for all the great responses... Sounds like I have many options... My hotel is Der Salzburger Hof-- Address is Kaiserschützenstraße 1, 5020 Salzburg which is east of the River near the train station. So I'm assuming that that means getting in/out will involve rush hour traffic...