I just got a '69 Roadrunner and the speedo bounces quite a bit, but the worse part is that it reads about double your actual speed. Drive down the street at about 35-40 miles per hour and it will bounce between 65 - 80.

I'm sure the bouncing could be the cable being bound up, dry or has a bad end but I don't understand how it can read double actual speed. I don't know what speedo gear it has in but it can't be that, no gear will make it off that much. Tires are 215/70-15's.

I assume it's the speedometer head but I was hoping that I didn't have to replace it.

Lube the cable and try it again. A sticking cable can do what you describe. Think about the cable sticking and then popping loose. Remember is coiled pieces of wire.

Thanks. I'll start by giving that a try.

Besides lubing your speedo cable, you also need a different speedo gear. If you know your rear end gear ratio and your tire diameter, you can select the right speedometer gear. It’s located where the speedo cable screws to the transmission. Check the tech archive under automatic transmission and you will find which gear you need. Sounds like your transmission is from a car with 2.94 gear and you have 3.55 gears. Which would create the situation you are describing.

I understand the speedo gear/tire size/rear end gear/etc but I do not believe that you can have a combo that would put you 100% off (my speedo shows ~double actual speed).

It wouldn't. At 2500rpm there is only 20mph difference in those gears. Not even close to double. What happens when a speedo cable sticks is that it acts like a spring. It winds up and then springs loose making the speedo jump.

I've removed the speedo cable and the ends look fine. I lubed the cable and there is no difference. In fact, at times the speedo will show 10-20 mph when the car is not moving. I guess I need to tear apart the whole darn dash to get the speedo out so I can take a look. I'm guessing some kind of return spring or the magnets...

That is the last thing to do!
Replace speedo cable first .

A speedometer can read as high as your saying if the transmission has let's say a gear for a passenger car like a 318 Satellite with a 2.45 rearend ratio and you actually have 4.10-5.38's. Seen it been there. Mark the tire and mark the driveshaft and get an idea of what gear set is installed by counting the # of turns the driveshaft spins to one turn of the tire.
Then pull the speedo gear from trans and put in correct one.
If all that checks out maybe you have a bad speedo head.

I'd consider replacing the cable - RockAuto doesn't have a B body list but the do show an E body listing - around $10

https://www.rockauto.com/en/catalog/plymouth,1970,cuda,7.2l+440cid+v8,1231759,interior,speedometer+cable,1440

I'd pull the cluster, remove the speedo and lube the pivot points for the needle on the speedo, any gears, and the flywheel assembly, with a paint brush very lightly dipped in wd 40. This is almost like jewelers work, lubing the insides of a watch. And I'd make sure the speedo cable interface on the back of the speedo isn't gummed up with 50 year old grease, or dry as a bone. It's real easy to ruin the face of a speedo with just the natural oils on your skin and fingers, never mind a lubricant, so judicious work is called for. I use a clean artists brush to dust off the face of gauges, so as not to smudge them. You can sort of bench test it by connecting the cable to a drill.

When you removed the cable to clean and lube it did it spin easily or did it hang up at all?

My 75 Dart does the same thing even after putting a new cable in it. I think it is the speedometer that might be the problem. Does the odometer work right? If you go down the road and watch it and the mile markers, is it close to right, or not? The odometer is gears and the speedo works off of magnets.

Yes, I forgot to mention that. The odometer works fine and is accurate to distance traveled, it does not match the speedometer. I agree with you, it's the speedo itself.

It's not the cable or the gear in the transmission.

Might just be dust and other particles that are getting between the rotating parts inside the speedo causing the problem. Realize it is a lot of work just to take it out and see, but you may be able to clean it out and then run it down the road with the speedo hooked up but not installed in the dash just for a tryout.

you could hook a cable up to the head while it's on the bench and use a drill to spin it in reverse [i think] to see if the symptom persists before putting it back in the car.

With this information them yes I agree it's a speedo problem.
Ron