I'll be in Peloponessus in a couple of weeks, and I am trying to get a rough idea of actual, real-world driving times.

I expect I'll want to absorb the scenery, make brief stops for snacks, and periodically get lost, too! Racing in fastest possible time isn't the goal.

The routes I'm trying to estimate driving times include:

1) Naufplio to Monevassia via scenic coastal road, taking it slow and stopping for scenery. Seems about 200 km.

2) Monemvassia, through Gytheio, to Geroliminas (southern tip of Mani peninsula). Seems about 120km.

3) Naufplio to Ermioni (to catch ferry to Hydra). Seems about 90 km over secondary roads. Here I actually DO want to keep the driving time short, since I'd be retracing road back to Naufplio.

You could try entering your routes at www.mappy.com or www.viamichelin.com.

My impression is that they both underestimate a little.

It's been a while since I was in Greece, but I remember most roads on the Peloponnese as narrow and winding, and I would think that would be particularly true in the Mani.

Mappy doesn't include greece, but the ViaMichelin site is helpful.

Probably I should add at least 50% to Michelin's claimed driving time to cover stops for scenery, snacks, etc.

Definitely include time for snacks, etc. I think these sites tend to presume that you get into the car and drive until you reach your destination.
I'm not as familiar with Viamichelin as with mappy.com, but mappy sets out the trip in minutes, and there is not a single minute allotted to stops of any kind.

We did a similar trip last year.
As I recall we left Nafplion at 9:30, spent an hour in Argos viewing the ruins there and got to Sparti at 3. Unfortunately we arrived at Mystras just as it closed. From Sparti it took us an additional 2 hours to get to Monevassia along the coast road. Apparently there is a better road which doesn't meander through every possible little village, but my navigator is slightly challanged (all those Greek signs you understand) There are certainly many other things to stop for other than snacks on this route.

We travelled from Monevassia to Gytheio where we spent a couple of hours and then went on to Aeropoli for the night. It is possible to head into the Mani that day but the drive can be a bit challanging at times and there is lots to see, so starting fresh the next morning seemed to be a good idea.
Despite its remote location there was a remarkable amount of RV traffic in the Mani and you don't go very fast. Areopoli through the Mani and back to Gythio took a full 8 hours. Of course we stopped a lot, particulary at Vathey where we spent two hours climbing throught the abandoned tower houses.

Expect the trip to take a lot longer than you planned.

Always allow more time than you think. It's very easy to find something that you didn't know existed (not in the books) and also typically, greek roadsigns can leave somthing to be desired even if they are in English (i.e. after the junction you want.)