Dialing in Speedometer/Odometer

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dibbons

dibbons

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From past forum discussions, I came to realize that we don't really have a speedometer "expert" on this forum, but I thought I would throw this out there anyway. My speedo was bouncing around wildly, but was restored and now is solid/steady as a rock. I have 25.9" tall tires with what I believe to be a 2.71/2.76 ring and pinion. The 904 has recently been rebuilt.

According to mile marker check on the highway, traveling 100 miles is reading 106 miles on my odometer (6 divided by 100 = 0.06) That discrepancy is easy to figure out, the odometer reads exactly 6% too high.

According to the neighborhood "courtesy" radar checks (i.e. before school zones), at an indicated 30 MPH on the speedo I'm actually only traveling about 25 MPH. That is a speedo error of a more than 16% (5MPH divided by 30MPH = 0.1666) On the freeway, at an indicated 70 MPH on the speedo I am still being passed by traffic, so at that reading I figure I'm really only traveling about 60 MPH. Even at an indicated speed of 80 MPH I still get frequently passed, so I must be doing around 70 MPH. If I can find a "courtesy" radar check on the highway, I can better pin down the exact mismatch between indicated and actual velocity.

To point of my story and the mystery I have in mind (assuming the only problem mechanically wrong is the wrong speedo/transmission gear color) is how/why the odometer is off by 6% and the speedomenter is off by 16%. I would have thought the error percentage would have been equal.
 
They made more than one drive pinion for gear ratios.

If you can find one for a 4.56 gear let me know. I can't find one. Mine is for 4.30 gears so I live with the speedo being off a bit.

I'd suggest you have the wrong drive pinion.
 
dibbons said:
From past forum discussions, I came to realize that we don't really have a speedometer "expert" on this forum, but I thought I would throw this out there anyway. My speedo was bouncing around wildly, but was restored and now is solid/steady as a rock. I have 25.9" tall tires with what I believe to be a 2.71/2.76 ring and pinion. The 904 has recently been rebuilt.

According to mile marker check on the highway, traveling 100 miles is reading 106 miles on my odometer (6 divided by 100 = 0.06) That discrepancy is easy to figure out, the odometer reads exactly 6% too high.

According to the neighborhood "courtesy" radar checks (i.e. before school zones), at an indicated 30 MPH on the speedo I'm actually only traveling about 25 MPH. That is a speedo error of a more than 16% (5MPH divided by 30MPH = 0.1666) On the freeway, at an indicated 70 MPH on the speedo I am still being passed by traffic, so at that reading I figure I'm really only traveling about 60 MPH. Even at an indicated speed of 80 MPH I still get frequently passed, so I must be doing around 70 MPH. If I can find a "courtesy" radar check on the highway, I can better pin down the exact mismatch between indicated and actual velocity.

To point of my story and the mystery I have in mind (assuming the only problem mechanically wrong is the wrong speedo/transmission gear color) is how/why the odometer is off by 6% and the speedomenter is off by 16%. I would have thought the error percentage would have been equal.
Click to expand...
 
easy way is to go on mapquest and get the mileage between two local points nearby. Then drive it an see what
your odometer says. calculate the error example, if you read 10 percent more than mapquest says you need
a speedo gear with 10 percent more teeth, that would slow the cable down by that amount. the speedo gears
come in different colors, find the closest one based on the gear presently installed. Keep in mind that your
speedo is probably not very linear, different errors at different speeds, so split the difference or get a GPS.
 
Mph apps are free on most phones. Go download one & check it.
 
Truth be told I believe they never were accurate when new.
 
Odometer is direct drive thus dependent on correct gear in the trans for its accuracy. There is no direct drive in the speedometer so that needle could report any mile per hour while you travel a million miles. It is actually 2 instruments in one assembly that were once calibrated to work together. One reporting the cables revolutions only, the other reporting the cables rpm.
So... with the correct gear in the trans, odometer will be correct due to its direct drive. Then the speedometer can be calibrated to correct MPH should it be wrong.
Trivia, back before radar , police/highway patrol vehicles had routine speedometer check/recalibrate. Why? The fact that they are not direct drive means they could begin correct and slowly change.
I've seen only 1 speedometer that had a dated "certified" sticker on it.
 
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Here's a chart, from Year One, that may help. --
MOPAR - Speedometer gear chart..JPG
 
Those charts above ^ are a good starting point but due to tolerances can still be off a little. I've found I almost always need to go 1 tooth higher or 1 tooth lower than the chart says.

Dibbons do you have a GPS or know someone that will loan you one? I have yet to see a GPS that doesn't show the MPH. Might make things easier
 
Yup, I do mine with GPS, Garman. They have the speed on the screen. I tried the Apps, but they seem to be slow and slightly behind in catching up with your speed as you move. The chart gets you within a reasonable tolerance.
 
RedFish said:
Odometer is direct drive thus dependent on correct gear in the trans for its accuracy. There is no direct drive in the speedometer so that needle could report any mile per hour while you travel a million miles. It is actually 2 instruments in one assembly that were once calibrated to work together. One reporting the cables revolutions only, the other reporting the cables rpm.
So... with the correct gear in the trans, odometer will be correct due to its direct drive. Then the speedometer can be calibrated to correct MPH should it be wrong.
Trivia, back before radar , police/highway patrol vehicles had routine speedometer check/recalibrate. Why? The fact that they are not direct drive means they could begin correct and slowly change.
I've seen only 1 speedometer that had a dated "certified" sticker on it.
Click to expand...

I worked on Police cars in Orlando FL, their speedometers were all certified, except for the Detectives who had unmarked cars. Can't remember if they were dated or not. To the OP, your odometer is about as close as you can get. The rebuilt speedo sounds suspect... Have someone with a newer car drive 70 so you can follow and check yours if you don't have a gps or app.
 
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