speedometer on 1993 Chevrolet Blazer

i have to go 30m.p.h. in order to go 25 m.p.h.
i don't really know how fast i'm going.
what could be causing this?

Asked by for the 1993 Chevrolet Blazer
Is this something that has just started or has it been going on for a while? Changing the transmission, final drive, or tire size can affect the accuracy of the speedometer. If everything on your truck is original and the speedometer is still off, the fault may be with the speedometer itself.