Some years back (could be as much as 12 years ago) I installed 3.91 gears in my car. Many times in the past I have used the charts in places like Brewer's (just for example) to determine the number of teeth of the speedometer gear and I was always able to get the speedometer accurate to within 1 MPH. My car has P25560/R15 tires which are 27.05" in diameter. The chart said use a 39 tooth gear (at least I thought it did-that could have been a mistake). I installed it properly with the little line of the 39-45 pointing towards 6:00. I could have sworn when I first did this it was accurate but now I am not too sure since it was years ago.
Recently I noticed that the speedometer seemed inaccurate. I checked it with the app "Waze" and indeed it is off by about 10%. When it says 77 MPH I am actually doing 70MPH (give or take 1MPH). So this time instead of using a chart I used several speedometer gear calculators that I found on the internet. One thing I needed to know was how many teeth are on the output shaft of the transmission. According to Brewer's all '66 and up transmissions (manual or auto) have 13 teeth on the output shaft. I don't know what year my transmission is but I highly doubt it is '65 or older. I have a Passon 4 speed OD so I am assuming it also has the 13 teeth. I know for a fact it was accurate when I had the 3.23 gears. I checked it with my Garmin GPS back then. Anyway, each calculator said I need a 38 tooth gear. So I bought a 38 tooth gear and installed with the 33-38 line pointing towards 6:00. The initial problem to me was, would not I have had to use a larger number of teeth to make the speedometer show a lower number of MPH? Although it didn't make sense I went ahead and finished installing the 38 tooth gear. Now here is the strange part. Nothning changed! It's still off by 10%!! It still reads 10% too high. Something should have changed. First thing I did was I double checked the teeth on both gears. Yup, the first one has 39 teeth and the second one has 38 teeth. Then I tried to count the teeth on the output shaft and it only seems like 10 or 11 teeth but it is very difficult to see since I don't have a lift. I could easily be wrong about that. What am I doing wrong? Is it possible the speedometer went bad and no matter what gear I install it will always be off by 10%? I am totally stumped on this.
Recently I noticed that the speedometer seemed inaccurate. I checked it with the app "Waze" and indeed it is off by about 10%. When it says 77 MPH I am actually doing 70MPH (give or take 1MPH). So this time instead of using a chart I used several speedometer gear calculators that I found on the internet. One thing I needed to know was how many teeth are on the output shaft of the transmission. According to Brewer's all '66 and up transmissions (manual or auto) have 13 teeth on the output shaft. I don't know what year my transmission is but I highly doubt it is '65 or older. I have a Passon 4 speed OD so I am assuming it also has the 13 teeth. I know for a fact it was accurate when I had the 3.23 gears. I checked it with my Garmin GPS back then. Anyway, each calculator said I need a 38 tooth gear. So I bought a 38 tooth gear and installed with the 33-38 line pointing towards 6:00. The initial problem to me was, would not I have had to use a larger number of teeth to make the speedometer show a lower number of MPH? Although it didn't make sense I went ahead and finished installing the 38 tooth gear. Now here is the strange part. Nothning changed! It's still off by 10%!! It still reads 10% too high. Something should have changed. First thing I did was I double checked the teeth on both gears. Yup, the first one has 39 teeth and the second one has 38 teeth. Then I tried to count the teeth on the output shaft and it only seems like 10 or 11 teeth but it is very difficult to see since I don't have a lift. I could easily be wrong about that. What am I doing wrong? Is it possible the speedometer went bad and no matter what gear I install it will always be off by 10%? I am totally stumped on this.
