Go to Mauville city's bikeshop (on the top right side), then ask Rydell for a machbike (not an acrobike) and he will give it to you.
It is mach bike because it travels fast unlike archo bike.
in mauville city your choices are a mach bike (very fast but hard to turn) acro bike (a bike that is not very fast but easy to turn and you can do stunts with it)
By using the fast bike that you get given to advertise with
You have to get the fast bike from the shop, then go back to where the mudslide was, gain some speed and you should be able to get up there.
You can get it in the bike shop in Mauville City.
acro bike
you use a mach bike to go over the holes really fast, but you have to back up little and then go over with out stoping
the stuntbike in ruby is called an acro bike and you can get it in mauville city
Get an Acro Bike and hold down B while moving.
You can get up the mud slopes using the Mach Bike at full speed in Pokemon Ruby. You can get the Mach Bike by talking to Rydel in the bike shop of Mauville City.
Not as fast as i am!
not that fast