You could equally well Google horse speeds. Get a sense of walk, trot, canter, gallop. With that, you might look at the distances involved. You don't say what those are, in your original post, but a horse can't gallop forever - and you kind of have a "do you get wetter walking or running in the rain?" question here.
Side trivia: Jingle Bells/Sleigh Ride talks about a horse with "two-forty for his speed" which was a reference to the horses pace for a mile. That's a very fast sleigh.
Another thought you might consider, depending on how granular you want to be, or how much you want to focus on the rain ride, is how will the horses respond to the acid rain. Riding rolls should be much harder if they're getting burned as they run. What shelter do they see that they could run to? Do they just drop and roll in the dirt to relieve the pain?