We will be driving across Spain May 2017. We will rent a car in Barcelona and drop off in Madrid. What do we need to rent or do to pay tolls on this journey? Will we know the amount of toll we are paying?

You stop at a barrier obtain a ticket when entering the road.
When leaving you will encounter a man/woman in a booth. Give them the ticket and they will tell you how much you have to pay.
No tricks no machine to be hired.

You can easily avoid the toll roads if you plan ahead and have the time. The main highways between Barcelona and Madrid is typically heady with truck traffic, especially as you get closer to Madrid.

If you are going to drop the car off at the airport (highly recommended), you can take the R2 as you approach Madrid. It will take you to the airport, avoiding the truck and commuter traffic. From the airport you can take a taxi into Madrid. The rate is fixed at 30€.

On viamichelin.com you can check boxes for "no tolls" and other options, and see where those routes are. Sometimes they are better than the obvious ones, either faster, or at least - if not faster - more pleasant, or more scenic.

With viamichelin.com, you can also see what the tolls will be.

Have you consider taking the train?

I would not drop the car at the airport unless that is your final destination. I have twice dropped my luggage at my hotel, refueled the car and dropped it at the car return by Atocha train station.

Cheaper and less time consuming than going to the airport. Unless you have a problem driving in cities.

Great advice from all and very much appreciated. We are trying to put together a self-guided winery tour from Barcelona to Madrid. We have 6 nights to spend in hotels before arriving Madrid to spend 5 nights. Is it reasonable to stay Zaragoza, Logrono and Burgos and enjoy our time? Maybe one, three and two nights; respectively.
Any ideas would be very helpful.

If your interest is wine touring on the way to Madrid, then you can start with the Somontano wine region in Huesca (www.rutadelvinosomontano.com). The Hotel San Ramón del Somontano in Barbastro makes for a good stop for the night.

After that you can head to Logroño or Laguardia for a couple of nights, then down to Peñafiel in the Ribera del Duero for your last two nights, staying at the Hotel Af Pesquera (www.hotelpesquera.com). Madrid's a two-hour drive.

Remember that you will need to have reservations if you plan of doing any wine actual touring and tasting. Some wineries in the Rioja do have walk-in wine bars, but most hours are limited. You can read Maribel's Guide to the Rioja for a little more information.

Excellent information! I will check out all the things you listed Thank you for your time.