the speedometer is separate from the fuel gauge. You might need the dashboard wiring repaired. My 2002 Buick is having problems with the lights in the dashboard and it will cost my $400 to replace it. I'm waiting til it all goes out for more than 48 hours.
If it's just the speedometer and not the lights, a wire contacted to your axels could be lose. Call the carlot or repair shop you go to and ask their advice before bringing it in.
Good luck!