2013-11-16 Cambridge via Duddenhoe End

Currently 48/16 on SS will I need to change the rear at all?

That's quite a high (3:1) ratio but if it's OK for single speed it ought to be OK fixed. If anything, I'd use a lower gear for riding single speed because you can always coast downhill, whereas on fixed you might want a higher gear for the descents so you don't spin out.

Yes, it's too high. Running an unvariable gear that high will give you knee damage in the long run. Go to 42/16 or something similar (around 70gi). Most other riders who are fixed will be around that ratio. 48/16 is gearing for the track, not for the road.