We have a simple line of gates which are connected to redstone. You run the horse at the line of gates, and you run over a pressure plate just before the first gate. This opens all the gates, and then closes them again after a tick of redstone power.

You can then measure how many single blocks your horse has travelled in that second, by seeing how many gates you got past before getting stopped.

Its a simple, easy and consistent way to measure speed which can translate directly into how long it'll take you to travel along our nether. 2000 block journey, 10m/s horse? 3 minutes and 20 seconds trip.

_________________
Petra Ravnikaar of The Veil

There also was a mod that showed Horse Speed, I will update if I have the link.

_________________
Hasan 'Darius V' Stark

Protector of the Vahnic Exploration Company
Ilkhan of Greater Karastan-Laghima

Ah, Minecraft engineering always impresses me. I'd never heard of the horse speed measurer, and it's little obscure things like that that show the potential and breadth of things that can be engineered in this game of blocks. I don't know why I didn't think of using pressure plates like that, since I have some engineering/programming understanding and logic, but now it seems obvious. I'm just not entirely familiar with redstone and I didn't know you could line them up with time, like seconds; I thought it was just those ticks. Anyway I should've thought of this because in my physics class we just did a very similar lab measuring the speed of little cars.