Last night, I was driving home from work with my girlfriend following behind me in her car. I thought something was odd because my speedometer read about 55 mph yet I was passing most everyone on the road. So I give her a call to ask her approximate speed. Before I get close to asking my question, she asked why I was going 75mph on the onramp. So yeah, this speedometer is waaaay off.

Now, I expected the speed to be somewhat off - I've got P205/70 tires mounted on Virgos and I installed a 120mph speedometer from a sedan (this is an '84 245). But I don't think this combination would cause the speedometer to be 20mph off actual?

I've also got the bouncy speedometer needle, so I know I need to lube or replace the cable for that. There was a problem with the original (80mph) speedometer not registering the correct speed when I bought the car and one of the receipts from the previous owner shows that the manual (4spd+OD) transmission was replaced.

I'm mostly concerned about accurately documenting the mileage as I perform maintenance (and mpg) but it would be reassuring to know I really don't drive as slow as the speedometer indicates.

Besides tires and speedometer, what else could be the culprit here? Could there be an issue with the transmission? The cable? Any insight on this matter would be greatly appreciated! :)