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.
Hairspring tension has a greater affect at low speeds, magnetism at high speeds. I find strong magnetism and hairspring strength hold the pointer better throughout the range, less bounce. Of course, needle balance also plays a large role. Kudos on the tech article.
Thanks! You're welcome to add some tips on how to do this...I haven't played with magnetism at all, how can a guy do this with stuff we might have in a home shop?
To build an appropriate magnetizer is cost prohibitive in comparison to just sending a speedometer in for service. It is worth noting that they do lose magnetism over time and compensating with the hairspring can shift the calibration. Really, it is far simpler to have it on the bench top calibration machine, but I digress. It is also worthy to note that most speedometers are calibrated to the same ratio and the cable transmission gearing is responsible for any tuning necessary. I know everybody is putting together some awesome conglomerations, but the original idea was "uniform ratio for the delicate part" and gear cutting for the rest.
So do all these old second hand speedo's we buy from the cl***ifieds, need calibrating. Can we just go into an instrument shop, with the speedo, tell them what trans, diff ratio and rear tyre height, and can they supply the appropriate cable and drive. Not that simple I presume.
There used to be a guy that sold these little gizmos on eBay that corrected for speedo gear/ rear end gear changes. You'd drive say 100 mph according to your speedo, and have a pal with an accurate speedo drive behgind you and notice how fast he was going, let's say 68 mph. You'd email this eBay guy and he'd set you up with something that would get you close. Just a little gear reducer/increaser that hooked up between the trans case and your speedo cable. I was going to get one, then I got a GPS app on my phone and now I use that.
The speedo calibration thing has been a constant thorn for me over the years as I would change tires, gears, etc. My OT 69 Frankentruck with the 700R4 was particularly bothersome attempting to pull everything together. But with my 49 Fleet I've got a Haneline cluster. Haneline can hook you up with a company that makes a small under dash gps unit that connects to the speedo ... end of the calibration PITA for me ... and I don't mind that at all. It ain't traditional but it uncomplicates my life.
Here is a source of info for speedo correction: http://www.yotatech.com/f116/mechanical-speedo-correction-box-install-pics-195986/ Nick
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