Here's some more info.   I've attached a photo of an adapter that will make the speedo read something different, they're custom made by speedometer shops and used on trucks etc. when they change tire size.   In the late 70's a similar adapter was sold so a mph speedo would read km/h.  The drawback with those is the odometer will also read km's, so making the odometer read more than actual.  Do you see a similar adapter on your speedometer cable where it connects to the transmission?

This particular adapter was made so a vehicle with a km/h speedometer would read mph.     If your car is reading km/h on the mph scale (with no external adapter), you can either fiddle around with the drive gears in the transmission, or use an adapter exactly like this.   You may be able to see in the photo, it's stamped .6250, which reduces the cable so the speedo will read mph on the km/h scale.      (1 km = .621 miles)

 

Converted the gear in my turbo 400 in the 80's, never quite read right, then about 5 years ago got serious and we matched tire size and the gear to make it read true.
Tried it with a police car through several speed zones and we were "close enough" not that she knew I was pacing her
The '69's odometer's on it's second go round now, not sure what the true count is, but says 18 thousand and change, but that's a mix of miles and klicks over the 43 years I've had it.

Greaser wrote:

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Wouldn't the easiest way to know what speed your doing in the old boat

is to get a cheap GPS that has the satallite speedo feature built in?

http://www.thefind.com/sports/info-gps-speedometer

That will tell you what speed your doing and have it placed somewhere on the dash with minimal or no wiring or modifying anything?

 

I don't know that much about them,but I have also seen an app on a cell phone

that tells you how fast your going in a car....and it was free.

http://www.windowsphone.com/en-us/store/app/free-gps-speedometer/d1cf4f3c-4939-e011-854c-00237de2db9ehttps://play.google.com/store/apps/details?id=com.codesector.speedview.free&hl=en

Seems to be a lot of GPS Speedo that don't require any wiring out there now...I'll leave my speedo as is and

look at getting one of these gizmos next spring for my 63.

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Coincidentally I have been doing lots of work programming Garmin GPS in the last couple of weeks for a work function and think using one of these would be a good way of understanding exactly what your speedo is reading.  There's no work to it, just plug it into a power source and it calculates your speed based on your position in relation to the satellites.

During testing I had the opportunity to check the speed on the GPS against one of those radar trailers that police use to display your speed, and have found the GPS to be pretty much bang-on with the radar. 

I have also noticed that not all cars (new cars with OEM tires and wheels) display the correct speed on their speedometers - some are correct, others tend to read higher than actual speed.  Just FYI.