'77 DL speedometer

[02Rubaru]

So I've got a 77 Subaru DL wagon with the single range 4 speed and currently the speedometer is off about 10 mph at ever speed.  So no matter how fast the speedometer says I am going, I am going 10 mph slower.  Ever since I've owned it, it has been this off, and at first I thought it was the bigger wheels the PO had redrilled but I found a stock set locally and installed those with brand new tires and its still off.  Then I read somewhere that the speedo cable could be frayed causing it to read incorrectly so I bought one of those and installed it, but its still off by the same amount.  So I was wondering if anyone had any ideas as to what my problem could be?