I assume that you are using GPSSerial with an external GPS. The speed is provided by the GPS not by the library. You can see what the GPS thinks the speed is by looking at GPSSerial.GPRMC, speed is item 7.
$GPRMC,hhmmss.ss,A,llll.ll,a,yyyyy.yy,a,x.x,x.x,ddmmyy,x.x,a*hh
1 = UTC of position fix
2 = Data status (V= fix inValid, A = fix Active)
3 = Latitude - ddmm.mm
4 = N or S
5 = Longitude - ddmm.mm
6 = E or W
7 = Speed over ground in knots
8 = Track made good in degrees True
9 = UT date
10 = Magnetic variation degrees (Easterly var. subtracts from true course)
11 = E or W
12 = Checksum
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