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Definitely do the Colle delle Finestre to Susa
How's the climb and, more importantly, the descent into Susa. I don't much fancy steep gravel/mud descent on even 32mm road tyres but if it's not too sketchy it sounds like fun.
atz-
It's not sketchy, but a good 10km of 10% gravel I think. I did it in 2017, but they included it in the Giro 2018, so I believe it has been resurfaced two years ago (hard packed gravel and such), although the pros typicall do this gravel bit as a climb.
From memory, the gravel bit is only on the Susa side, from the top to this point. Personally I would do it on 28mm road tires with touring setup, so you should be fine, but take a look at it on Street View and decide for yourself. It probably won't be a record breaking discent.
EDIT: The views will make up for any discomfort/concern during the descent on gravel.
giofox
Road, gravel or MTB?
The road from Briancon to Sestriere can be busy, but wide enough. There is a tunnel on the Sestriere side, but bikes go on the old road from memory.
Definitely do the Colle delle Finestre to Susa, simply stunning.
If you are on a gravel/mtb, the Strada dell'Assietta (Strada Provinciale 173) is also riveting. Basically what I am suggesting is the Torino-Nice Rally route, in reverse.
https://ridewithgps.com/routes/16767562