Wagon Help

So long story short, in the next game we will be doing more merchant adventuring. My character being able to build automatons has made a unsleeping, never tiring horse to pull a wagon. Only it seems a wagon is limited on its speeds. Is this based on the wagon itself or the horses that pull it? Also how does it work for a auto that just does maximum effort all the time; does it move at a slower pace when pulling more stone or is that a limitation of the horses once again?

If it is a limitation of the wagon what is a fun ways could I do to make something that isn't as limited but doesn't cost 50k to carry a lot of merchandise and people?

I would assume that it is a little bit of both. Without knowing much about your specific construct I would say the main advantage comes in the fact that as the mecha-horse never needs rest, you could rule that it can move twice as far in a day's travel. This would assume that you have characters taking turns to guide it through the night, etc. Ultimately, even if the horse isn't limited, the passengers are. They probably need lunch breaks, time spent "out of the saddle" as it were, and other biological issues. 

That said, if the horse can just keep on walking forever, then sure, it could definitely achieve a farther "per day" travel rate. 

How about:
-Can do a forced march all day and every day
-Counts as 4 heavy horses for wagon speed
-Wagon load doesn’t effect speed.

If any of the limitations are because of the wagon then it wouldn't matter my machines stats. I would understand that wagon still having a limiting stone weight but its the movement speed I am wondering about. Can a wagon only go 60 feet a turn without breaking apart? If so then I would have to build a new wagon.

If not that upping that speed and weight is all I need. If I could make something that moves 240 feet per turn then even with breaks you are still moving 3 times faster than a normal wagon could without nighttime travel. In reality moving at night is just a bonus not something I would really use or optimize for unless really needed.

What me and the DM worked out a while after I posted this. Still would love some feedback as the more angles we can have the more options we can poke around with and get the best result for both of us. 

"The horse is pretty much 4 heavy horses in one. The wagon, specially made, is reinforced enough to warrant the ability for another branch of weight. The horse can carry a wagon like 4 horses on a wagon at 640 [stone] at 60 feet per turn, or 1240 [stone] for 30. [W]ith a second horse, it comes to 60 feet. with 4 horses, it comes to 120 feet. however, 1240 stone is as much as you can stack on that wagon"

He has it as 3 times the cost of a normal wagon and can only be made by Dwarfs. We figured if you want to have it made, because it wasn't going to be just laying around, it would cost double wages for a spcilist. 600(Cost)/120(master craftsmen and his people can make in a month)=5*75(cost of a armorer)*2(figured it would work something like magical items)=750+600=1350 GP total.