By Estimation and Gearing.
L._. : I recently purchased a 1972 Malibu that I am trying to get back into decent shape. A big problem is that the speedometer is at least ten mph off. How can I get it back to the correct readings? It is an automatic, and I have no idea what the rear gears are or if every component is original.
C.H. : The first thing you have to do is get the odometer as accurate as possible. To do this you have to establish the error you presently have. Drive your vehicle through an accurate speedometer check zone on the highway. Most states have these on an open stretch of highway near urban areas. If not, most states have mile markers that are fairly accurate. It is best to have someone with you to note the readings you tell them. As you go by the first mile marker note the last couple of digits on the odometer, say 12.7, then go ten miles and note the odometer reading at that point. If it reads 22.9, that means your odometer is off 2 tenths in ten miles. Divide .2 by 10 which equals .02 or 2%. Relate that to the teeth on your driven speedometer gear. You know in the above case that you have to slow down the rotation of your speedometer cable, so that means you increase the teeth on the driven gear. If your gear has 32 teeth, adding one tooth is 1 divided by 32 or 3.1%. In this case your 33 tooth gear will be more accurate but will have an error of about 1.1% in the other direction. The only way to get more accuracy involves different drive gear and driven gear ratios, which is more complicated than it is worth. Remember this is crude and not highly accurate, but is fairly effective.
If you want to have your speed indication more accurate you have to turn into a clock maker of sorts. First, find out your actual speed. The easiest way to do this is to go through a measured mile maintaining a steady speed. You need a stopwatch and a calculator to make it easy. Get up to your indicated speed (say 60 mph) and hold it as closely as possible through the one mile. Time yourself through the one mile. If your stopwatch shows exactly 60 seconds, then your speedometer is dead on, but if it is not then you have to calculate.
Let's say it took 1 minute, 3 seconds to go through the mile. Divide the 63 seconds into 3600 which will show your actual speed was 57.1 mph. Now, if you are brave enough, you can take your speedometer apart and lengthen the little spiral spring in the speedometer (not easy). The whole process is trial and error. If your speed is off in the other direction you would shorten the spring. You will get more accurate results the longer you go, provided that you can hold the speedometer on exactly 60 mph. In this case you would average the seconds. If your stopwatch read 2 min. 36 sec to go 3 miles, you would have 60 + 60 + 36 which equals 156. Divide this by 3 which equals 52 seconds average. You then divide that into the 3600 to give you the actual speed of 69.2 mph.
B.T. : There are 2 plastic gears in the transmission where the speedometer cable screws on. Take the speedometer cable off, remove the bolt holding in the round piece behind it, and pull it out. This plastic gear comes in many different colors, each of which have a different number of teeth. Your local dealer will be able to give you the correct color gear for your rear axle ratio and tire size. You will need to know what gears you have in the rear and the size of your tires. The tires listed by Chevy are the old F-70-14 designation, so you will have to find the closest tire size to what you are running today. Someone must have switched transmissions or rears in your car over the years.
Archivist: Tom Wilson
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