replaced speedometer driven gear with recommended 40 tooth and still not accurate...

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nitronut said:
Ouch. I sure hope not but who knows? The transmission shop here in Michigan didn't know tooth count /mph difference and referred me to this place...
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Just think about this: 3mph at what mph? 40 tooth to 41 tooth is around 2.5%. It slows the speedometer 2.5%. So 5mph = 4.9mph, 40mph = 39mph, 60mph = 58.5mph, etc... The math here is extremely simple.
 
Bad Sport said:
I didn't read each post in detail, but I didn't see clocking the housing mentioned. Is the housing clocked correctly?
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I'm going to double check that. I think so but I'm not sure... I've asked that question to Mancini, my transmission builder and another trans shop and they said it shouldn't make a difference. I've also read online that if clocked incorrectly it would put more pressure on the driven gear if that was the case. The gear numbers of the driven gear should be located at 6 0'clock - is that correct?
 
nitronut said:
I recently installed a new speedometer cable and the mph readings were off by 20-30 plus mph. Called Mancini Racing, gave them my tire size (P225R1595S) along with the rear end gear size (3.91) and was told I needed to install a 40 tooth driven gear, which I purchased and installed. (The old gear I replaced was a 23 tooth). It still seemed off so I used the speed app on my phone and now reads accurate to 10 mph and then jumps the speedometer reading 10-13 mph OVER my actual speed when driving faster than 15 mph. Is there a rule of thumb for speedometer speed readings per gear tooth? Do I need a 42-44 driven gear instead of the 40 tooth? Any thoughts? Thanks in advance...
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What is the speed that the speedometer is reading and what is your actual speed???

Of go on the highway and do a steady 60 MPH then pass a mile marker, start timing when you pass the mile marker and see how many seconds it takes to pass the next mile marker while maintaining a steady 60 MPH...

Then post it up and I can do the math to tell you what direction to go...
 
nitronut said:
I'm going to double check that. I think so but I'm not sure... I've asked that question to Mancini, my transmission builder and another trans shop and they said it shouldn't make a difference. I've also read online that if clocked incorrectly it would put more pressure on the driven gear if that was the case. The gear numbers of the driven gear should be located at 6 0'clock - is that correct?
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Look in the service manual and it will explain how to clock the housing for the number of teeth on your gear....
 
69_340_GTS said:
Just think about this: 3mph at what mph? 40 tooth to 41 tooth is around 2.5%. It slows the speedometer 2.5%. So 5mph = 4.9mph, 40mph = 39mph, 60mph = 58.5mph, etc... The math here is extremely simple.
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Math has never been simple to me (lol) but I get what you're saying. What would you do? The speedometer was working correctly before the engine/rear end swap and the shop that replaced the rear end gears swears it's a 3.91
 
krazykuda said:
What is the speed that the speedometer is reading and what is your actual speed???

Of go on the highway and do a steady 60 MPH then pass a mile marker, start timing when you pass the mile marker and see how many seconds it takes to pass the next mile marker while maintaining a steady 60 MPH...

Then post it up and I can do the math to tell you what direction to go...
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Thank You!
 
nitronut said:
Math has never been simple to me (lol) but I get what you're saying. What would you do? The speedometer was working correctly before the engine/rear end swap and the shop that replaced the rear end gears swears it's a 3.91
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Krazykuda has the right idea.
 
nitronut said:
I'm going to double check that. I think so but I'm not sure... I've asked that question to Mancini, my transmission builder and another trans shop and they said it shouldn't make a difference. I've also read online that if clocked incorrectly it would put more pressure on the driven gear if that was the case. The gear numbers of the driven gear should be located at 6 0'clock - is that correct?
Click to expand...
You clock the line on the gear housing with the corresponding tooth value to the dot on the transmission @6:00. The clamp will fall into the correct grooves if you have the thing clocked correctly.
 
69_340_GTS said:
You clock the line on the gear housing with the corresponding tooth value to the dot on the transmission @6:00. The clamp will fall into the correct grooves if you have the thing clocked correctly.
Click to expand...
Thanks a million. I'm going to check that out.
 
Tire Height Calculator - Wallace Racing

727 SPEEDO GEAR COLOR CHART.jpg


speedochart.jpg
 
If you have a Smart phone, go to the APP store & get a speedo app. Put the phone on your dash & compare what it reads, versus your speedo. Then you will know how far off it is.
 
Demonracer said:
If you have a Smart phone, go to the APP store & get a speedo app. Put the phone on your dash & compare what it reads, versus your speedo. Then you will know how far off it is.
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Did that last night. Spped is spot on until 25 mph then slowly reads faster than actual speed up from there. Example: 30 mph on gps reads 35mph on speedometer. 70 mph on speedometer reads 57 on gps. The odometer however is SPOT ON. Our highways are one mile apart and the odometer was dead nuts on. I don't know what's going on..
 
69_340_GTS said:
You clock the line on the gear housing with the corresponding tooth value to the dot on the transmission @6:00. The clamp will fall into the correct grooves if you have the thing clocked correctly.
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Checked it out and that setting is correct. The tire height is 25.5 inches. Took the car out again late yesterday afternoon . The speedometer is very accurate to 25mph (gps) and then starts creeping up faster on the speedometer compared to the gps. Reads 35 when doing 30. Seems like about 1.5 mph faster speedometer reading than the gps for every 5mph increase in speed. Speedometer was reading 70 mph and the gps was reading 59mph. The odometer was spot on though. Our highways are exactly a mile apart and the odometer was dead nuts on that regardless of speed. I don't know. Honestly, I'm more concerned about the correct mileage on the car but the speed difference has got me baffled.
 
nitronut said:
Did that last night. Spped is spot on until 25 mph then slowly reads faster than actual speed up from there. Example: 30 mph on gps reads 35mph on speedometer. 70 mph on speedometer reads 57 on gps. The odometer however is SPOT ON. Our highways are one mile apart and the odometer was dead nuts on. I don't know what's going on..
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The speedometer itself is not operating correctly. It's not the gear in the transmission.
 
CudaFactHackJob said:
To find actual tire diameter, go from the ground to the middle of your back wheel and multiply by 2.
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That's a waste of time. The tire manufacturer's numbers are close enough. A 225/60R15 is 25.6" diameter. Use that number. You won't be able to measure closer than +/- 1/4" anyways. Which is less than 1%.
 
69_340_GTS said:
That's a waste of time. The tire manufacturer's numbers are close enough. A 225/60R15 is 25.6" diameter. Use that number. You won't be able to measure closer than +/- 1/4" anyways. Which is less than 1%.
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Took the car out for a ride this morning with the gps just because. Everything was still the same. Accurate up till 25mph, and speedometer needle bouncing a little bit under 10mph. And the speedometer reading much higher than the gps after going faster 25-30mph and up. I don't know what happened but now the speedometer reading is right on the money at all speeds and matches the gps reading. The needle no longer bounces at 10mph and under. Smooth and accurate. I have no idea why. Thanks again for the help.
 
threewood said:
Your speedometer most likely needs rebuilt. I am dealing with the same issue. Using the correct gear, speed shows low according to gps speed. If I change the gear to show accurate on the needle it throws the odometer way off. There are two tabs that spin around a wheel which moves the needle.

With the correct gear in your car, google map the distance driven with the odometer and see if it is close.
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Working perfect now!
 
nitronut said:
I don't know what happened but now the speedometer reading is right on the money at all speeds and matches the gps reading. The needle no longer bounces at 10mph and under. Smooth and accurate. I have no idea why. Thanks again for the help.
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Perhaps when you disturbed the cable and put it back on, it picked up a slight bind which would keep it from turning as it should. I had similar issues when I replaced my speedo cable. The bouncing under 10 was probably the cable binding and release, bind and release. Anywho, glad ya got it.
 
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