You can change the precision of values written into the dump file with
the dump modify command. Also you can use the compute reduce
command to sum up a velocity component across all atoms (or a variable
formula to calc momentum if the masses are different, and then
sum it via compute reduce). They you can print out the value
of the compute with thermo output. This should be a hi-precision result
since its all done internally in the code.
Also you can use