21 kph (13 mph)
For this question, we’re going to assume that we’re going in an exact circle around the equator.
The circumference of the earth is 40,075 km (24,901 miles). There are 1920 hours in 80 days.
Simple math tells us that you’d need to travel at an average speed of about 21 kph (13 mph).
Let’s explore this a little.
Jules Verne wrote Around the World in 80 Days in 1872. One of the fastest ships in the world at the time was the Cutty Sark. It was able to travel at a consistent pace of about 15 knots (27.8 kph).
Around the equator, 78.8% is over water and the rest is land. This means that 78.8% of the voyage could be traveled at 27.8 kph.
78.8% of 40,075 km is 31,579km. At 27.8kph, this would take about 1,136 hrs. This would leave 784 hrs (almost 33 days) to travel over the remaining 21.2% of the distance (8,496km) over land. That amounts to an average speed of 10.8 kph (6.7mph). An average speed of 10.8 kph means that you’d have to travel 259km per day.
According to website Petticoats and Pistols, “a young horse in optimal condition can travel fifty to sixty miles a day in good weather over level terrain”. So, under optimal conditions, with a single horse, we’d be about 200km short per day from meeting our goal.
So, traveling by horse is not going to do it. A stagecoach could cover 60 or 70 miles in a day, so that’s only a little better.
How could it be done?
There were two modes of transportation over land that were possible at the time – balloons and trains.
Balloons could travel at the speed of wind, which is reasonably fast, but this was a time before the availability of liquid propane, so travelling long distances was impractical.
Trains could travel as much as 100 kph at the time, but of course, there were no trains that would go directly around the equator.
How is it done in the book?
According to the summary by Encyclopedia Britannica – the journey is a combination of trains, ships, and even elephants. Most importantly, the journey is through Asia and America in the northern hemisphere (roughly between the 20th and 50th degrees of latitude.
More important still was that the wager was lost (spoiler alert) and the travelers didn’t make the journey in less than 80 days.
How could it really have been done?
Take a very fast ship to the southern ocean and travel the shortest distance around Antarctica.