02Rubaru

PM sent about the wood grain interior

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?