How fast can you travel by horse anyway?

How fast could this horse go?

While working on my second historical mystery, From the Charred Remains, I came across a rather straightforward mystery of my own.  How long would it have taken to travel the fifty-plus mile trek from London to Oxford, by horse and carriage, in the mid seventeenth-century?

 I have some faint memory of an equation that claimed distance=rate x speed (and even worse memories of trying to apply that equation).  I don’t think that equation works, though, when you don’t know the weight of a cart, the strength of a horse, or the conditions of the roads. 

So I had to set some parameters. I needed the cart (wagon, really) to be able to carry two men and two women, along with two or three barrels or bags of miscellaneous supplies.  I needed the journey to take less than a day.   The wagon had to be decent, but more serviceable and sturdy, than luxurious. It had to be capable of traversing 50 or so miles of the muddy, unpaved London Road. Similarly, the horses had to be from a hearty stock, and affordable for hire by a journeyman. Not being an equestrian, a farrier, or a blacksmith (okay, let’s face it, I’m not even sure if I’ve ever even been on a horse), this has been a truly puzzling question. 

So doing a little digging into the Early English Books Online and a few other primary sources, I first learned what kinds of wagons would have been available to a London tradesman in 1666. Here, I relied mainly on woodcuts to show me pictures of how tradesmen conveyed goods.  Hackney carriages were available for hire, but those would not likely have been owned by a tradesman. Coaches (Berlins) were just coming into fashion, out of Germany, but again my tradesman would not have found such a vehicle suitable to his needs or budget. 

Wing / 1917:08 

As for the horses, I looked to Gervase Markham, a seventeenth-century self-titled “Perfect Horse-man,” who shared his “experienced secrets” on horse care and training. He mentions some different kinds of horses (or perhaps more aptly, the services horses can offer), including the “courier,” the “carter,” the “poulter,” and the “packhorse.”   

Unfortunately, throughout Markham’s lengthy 200+ pages of advice to the horse-challenged, I could only find one bit of useful information for my purposes.  He says: “In journeying, ride moderately the first hour or two, but after according to your occasions.  Water before you come to your Inn, if you can possibly; but if you cannot, then give warm water in the Inn, after the Horse hath fed, and is full cooled within, and outwardly dried.” He then went on to say something about applying copious amounts of “dog’s grease” to the horse’s limbs and sinews, but I think I wandered off the page at that point.

Then I needed to find out how fast two horses can even pull a wagon.  Throwing my question to the whims of Google yielded an oft-repeated response: a team can travel 4 miles an hour on paved or semi-paved roads. Horses can only travel a few hours at a time; so it looks like my fictional travelers will have to exchange horses several times at various coaching houses along the way. 

This would mean it would take my travelers 15 hours to travel from London to Oxford, which is FAR TOO LONG for the purposes of my story. Yet, I've always been extremely scrupulous in my attention to historical details. So my puzzle has resulted in another conundrum—bend the facts to fit my story, or bend my story to fit the facts? 

What to do? What to do? What would you do?