My 67 Coronet street car - - the speedometer is pretty much dead on, but the odometer is approximately 19% off (tacks on more miles than actual)? What causes this?

My guess is the magnetic pick-up / bushings needs rebuilt. I think the miles are a result of the cable directly turning a set of gears.

But the speedo needle actually moves by a closely toleranced magnetic assembly.

Does the needle jump around? Mine eventually broken and had to get the bushings repaired.

*** OOPS, re-read and I got it backwards, I guess for some reason the gear portion of yours needs attention.

I'd get the right speedo pinion into the trans (more teeth) to get the odo spot on (check on interstate mile marker signs which are pretty accurate) then at that point if the speedo is inaccurate (checked at a roadside digital MPH sign) then the speedo head needs to come out & delivered to a speedo specialty shop & they'll most likely want to know the percentage of error (& which way). Put a piece of flat stock on top of a rear tire and measure the actual dia to ground then with your rear gear ratio # go to the archives chart & find the tooth count you need & get one & install it & go from there. #1 need the right pinion to mechanically make the odo correct then see if that makes the magnetic gismo in the speedo correct also

Why not just use the tire size to calculate the diameter? I can't image the vendors would be off more than +/- 1/8" which won't make a difference.

I would hope their ability to make the tire is better than our ability to measure it using a tape measure.

http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireMath.jsp

Yes measuring rollout with a tape measure would be dead on and you're only talking 7-8 feet but I did mine top to bottom and with the gear ratio in hand picked the right one from the chart & it made my odo and the speedo correct but I'm gonna listen to what Kunkel has been telling me for years and next time measure rollout. EDIT and John you just divide Pi into the rollout for the dia right?

Just divide the rollout in inches by 12 to get feet and divide that into 5280 to get the number of wheel turns in one mile.

but I'm going to want diameter (along with gear ratio) to use the pinion chart. How is figuring tire revolutions per mile going to help with that?

I don't trust charts, they round off numbers..I like to do the math myself. (can you say anal?)