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 mphoff. How can I get it back to the correct readings? It is an automatic, andI 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 aspossible. To do this you have to establish the error you presently have.Drive your vehicle through an accurate speedometer check zone on thehighway. Most states have these on an open stretch of highway near urbanareas. If not, most states have mile markers that are fairly accurate. Itis best to have someone with you to note the readings you tell them. As yougo 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. Ifit 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 yourdriven speedometer gear. You know in the above case that you have to slowdown the rotation of your speedometer cable, so that means you increase theteeth on the driven gear. If your gear has 32 teeth, adding one tooth is 1divided by 32 or 3.1%. In this case your 33 tooth gear will be moreaccurate but will have an error of about 1.1% in the other direction. Theonly way to get more accuracy involves different drive gear and driven gearratios, which is more complicated than it is worth. Remember this is crudeand not highly accurate, but is fairly effective.
If you want to have your speed indication more accurate you have to turninto a clock maker of sorts. First, find out your actual speed. The easiestway 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 yourindicated speed (say 60 mph) and hold it as closely as possible through theone mile. Time yourself through the one mile. If your stopwatch showsexactly 60 seconds, then your speedometer is dead on, but if it is not thenyou have to calculate.
Let's say it took 1 minute, 3 seconds to go through the mile. Divide the63 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 lengthenthe little spiral spring in the speedometer (not easy). The whole processis trial and error. If your speed is off in the other direction you wouldshorten 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 caseyou would average the seconds. If your stopwatch read 2 min. 36 sec to go3 miles, you would have 60 + 60 + 36 which equals 156. Divide this by 3which equals 52 seconds average. You then divide that into the 3600 to giveyou the actual speed of 69.2 mph.
B.T. :   There are 2 plastic gears in the transmission where the speedometercable screws on. Take the speedometer cable off, remove the bolt holding inthe round piece behind it, and pull it out. This plastic gear comes in manydifferent colors, each of which have a different number of teeth. Yourlocal dealer will be able to give you the correct color gear for your rearaxle ratio and tire size. You will need to know what gears you have in therear and the size of your tires. The tires listed by Chevy are the oldF-70-14 designation, so you will have to find the closest tire size to whatyou are running today. Someone must have switched transmissions or rears inyour car over the years.
Archivist: Tom Wilson