LiFePo4 Battery Issue

I recently bought a used 48V 20AH LiFePo4 battery for my eBike.  The seller said it had been used lightly for a couple of months and is practically new (which I know is probably not true).  

At full charge, the battery level indicator says 3 out of 5 battery level, but stays that way for the first 8 miles or so (I'm supposed to get 18-20 miles on a full charge).  

I'm afraid to try to ride the ebike and drain the battery (to see if I actually get the full 18-20 miles) because I don't want to be stranded half way to my destination and have to pedal a 90 pound bike another 10 miles.

I know the issue is not the battery level indicator, because it worked fine with my SLA batteries.  

Did I get a dud?  Is it not charging all of the way?  Any help or feedback would be great.

It stays 3/5 for 8 out of 18 miles, then starts dropping normally? Its probably the electronics in the battery (if it has any). Measure the voltage when its fully charged, and then again after doing a few miles.

you got to balance the cells