How old/strong are you?
I'm pretty strong but I can't pull 48:11 anymore (young had a 52 tooth). At cadence, downhill, I can manage 48:15 or if it is really, really steep 48:13, my cadence drops to 60 or below if I try 48:11.
Now if I add a hub motor to the mix I can pedal all day at 48:17 = about 34 km/h at cadence (95 rpm).
Chainwheels don't cost the earth, buy a 44 or a 46 as well in case the 48 is too great an increase.