1,000 RPM equals 60 MPH, 1,000 turns shows 1 mile on odometer on nearly all USA speedos back to mid-neolithic times.
You need of course to start with correct drive gears for your axle ratio and tire size...this can be checked on a chart in the '28-48 parts books.
Serious specialist shop does major recalibration by re-magnetizing drive magnets within drum.
Very minor fiddling can be done by adding or subtracting tension on the watch spring that returns needle and drum...there is a tiny lever in there with an obvious connection to the spring once you have dug in far enough.
Major drive correction if you have oddball tire size or cannot get proper gear set can be handled by ordering a ratio correction box, a little quick change box that goes between trans and cable...these are custom assembled to match your problem.
I am going to append my take on minor speedometer repair below, from an earlier post, and near the end there is linking to specialist shop with good info on measuring/calibrating what you have...they also do rebuilding and sell the correction gearboxes.
Here's my speedo repair technique: remove bezel and glass, get speedo out of case. Get a tin can or something that allows you to set the speedo down with face up and secure balance, and study how you are going to hold the thing so neither your fingers nor the liquids to be used can touch the face or odometer. That stuff is VERY delicate and easy to destroy.
Near the cable end, on what amounts to the speedo's main bearing, is a tiny brass plug. Get it out--I usually manage to extract them unhurt by threading a tiny machine screw into the cup--and extract the piece of felt within.
I then repeatedly flood this lube well with first kroil then WD40 as it frees up, sometimes liquid wrench if I feel something thinner is needed. You want lots of fluid to go in there and then leak out the bottom to dissolve and wash out the petrified lubricant and crud.
When the shaft will spin freely by finger twirl--you should be able to get 5-10MPH easily by finger--soak several times with carb cleaner and then brake cleaner to remove all those miracle solvents. When drained and dry, I fillerup with Marvel Mystery Oil, based on my scientific observation that the only real speedo tech I ever saw in action used something that sure looked like MMO. Stuff felt&cap back into hole, well soaked. Let drain, reassemble.
The multiple screwdriver tip sets you can get at discount stores contain a Canadian Robertson tip (square) that fits early Ford speedos and allows you to motor them up. You can also do a very rough test based on the supposed RPM of your electric tool, based on the speedo's calibration of 1,000 RPM=60 MPH.
Here's a ratio test page:
1.http://www.speedometershop.com/rep-pag.htm
http://www.speedometershop.com/rep-pag.htm go to "do your own ratio test" link
Following from post by SQUIRREL on the HAMB:
[FONT="]The speedo in my 55 chevy has a long history of not being quite right. In the old days, I would drive on a highway with mile markers, hold the speed at 60 on the speedo, and see if it took exactly 60 seconds to go a mile. Now that we have GPS, it's easy to see how far off it is at any speed.
I still use the mileposts to check the odometer reading, which tells me if the gears in the transmission are correct. I drive a few miles, and see how far off the tenths are. I use this to calculate how many percent fast or slow it is, and add or subtract teeth from the driven gear accordingly. For example, if the tenths are increasing an extra half tenth every mile, then the driven gear needs five percent more teeth. On a trans with a small gear (manual or th350) that would be one more tooth. on a trans with a large gear (like a th400) that would be two teeth.
Once the odometer is close enough then I look at the speedometer, compared to the reading on the GPS. This time mine was pretty close, it was reading about 5mph slow at low speeds, and a couple mph slow at around 75. I figured this means I needed to decrease the spring pressure, and move the needle a little to the right.
The needle can be moved by twisting it relative to the shaft, I held the shaft gently with needle nose piers and twisted the needle with my fingers (holding it at the reinforced hub).
The spring can be adjusted by moving the lever that holds the end. A little goes a long way! I just barely nudged it clockwise.
It seems to be much better now, it's still off at very low speeds, but I never look at the speedo below 25 mph anyways.
The electronic speed sensor is for the cruise control, and notice I added a Molex connector (from Radio Shack) for the gauge wires, so its easy to pull the cluster out to work on it. [/FONT]
